I would say that there essentially three reasons why England at least only made use of slave labour on a meaningful scale in her colonies and not at home, although the third flows from the first, really.

The first, and primary, reason was that Britain's economy was not particularly suited to the use of slave labour on any meaningful scale. Britain's economy was in many ways the first in the western world to achieve a rural labour market that exhibits some of the defining features of economic modernity: in other words, it was distinctly capitalist, and there was no such thing as an English peasantry by the heyday of the transatlantic slave trade. From the beginning of the 17th century there had been a process in England of privatising agricultural land had been historically open to use by the English peasantry to grow their own food and keep their own animals even if they often did not officially own it. This process accelerated massively through the 18th century, particularly from the 1770s onwards. Over the course of two centuries the English peasantry was gradually dismantled as the countryside was given over to the complete control of rural land-owning elites. Large estates comparable to the plantations of the American South were, however, still rare, and English agricultural remained focused primarily on food production. One of the driving factors in the industrialisation of the English North was that this new, marginalised rural population was growing significantly faster than the agricultural economy could provide employment opportunities for, creating a significant surplus labour force seeking work in cities.

Significantly since 1601, what we now call the Old Poor Law had made communities in England and Wales responsible for the maintenance and "correction" of their own unemployed and destitute residents, at their own cost. Not only would importing slave labour have created a significant mass of unemployed workers but it would have disrupted existing systems of coercive, cheap labour in the English countryside. In many parts of England the problem of labour surplus had been managed by creating compulsory local labour exchanges in which unemployed and impoverished members of the rural working class were compelled to work for local farmers who had the cost of employing them subsidised or paid in whole out of local taxation, paying the actual workers an often minuscule wage. In others the parish authorities reached an agreement with local farmers about the fixed allocation of jobs and set wages for those who did them to ensure there was enough work to go around. The introduction of another system of forced labour would have simply created an even greater population of white forced labourers and brought with it significant social, economic and political disruption to an economy that simply did not have need for the extra workers from the perspective of the employing elite.

It's worth keeping in mind that this problem of labour surplus was not unique to the English rural economy: it also existed in the slave economy of the southern United States. The explosive growth of racial slavery in the United States and Brazil was driven by the fact the climates of these countries were well suited to the mass production of labour-intensive cash crops, particularly cotton, with ample land to keep expanding the slave economy into. The invention of the cotton gin in particular was transformative to the American economy. It allowed the processing of cotton on a much more meaningful scale than anything that had been possible before which in turn meant that plantations owners could start planting more cotton, no longer burdened by lengthy processing times - that in turn created an explosive demand for more slave labour. Britain's industrial economy benefited from the existence of slavery in the United States, the Caribbean and India by virtue of being able to import raw materials for the industrial economy at a much lower price than would have otherwise been possible until industrialisation was thoroughly cemented and the imperial economy was sophisticated enough to respond to transatlantic supply shocks (as it did very well when the Civil War and abolition of slavery in the 1860s US disrupted the raw material supply). States like Virginia benefited by being able to supplement their more modest agricultural incomes by selling enslaved people further south to cotton-producing states with a much greater demand for forced labour, creating a vast and profitable internal slave trade.

The second reason is that from the very beginning of the slave trade the legal status of slavery in England was ambiguous and unclear. The English judicial system issued a series of difficult and conflicting interpretations of the law throughout the time Britain was a slave power - although slavery was unambiguously legal in the colonies, its legal status at home was in contention. As early as the 16th century an English court had found there was no legal foundation for slavery in the country, though that was concerned with the enslavement of other white people, whilst judgements in 1701 and 1707 had both suggested slavery in England might not be legally sound in the case of African people who converted to Christianity and, possibly, in general; a 1729 judgement reversed that ruling. Further confusion was introduced in a seminal 1772 ruling by the Lord Chief Justice that declared once an enslaved person entered England, he or she could not be forced to leave the country again without freely given consent to make the journey. Although that did not outlaw slavery itself, the ruling attacked the central logic of slavery as early judgements with the exception of the 1729 ruling had done: it held that there was no legal basis or foundation for the existence of slavery in Britain and that it could very, very easily be abolished or limited through legislative or judicial action. The Lord Chief Justice was absolutely right in this - there was no law that positively described a condition of slavery in England. That judgement seems to have seriously chilled the state of English slave holding such that from the 1790s onwards, the domestic slave trade appears to disappear from the historical record, and all enslaved people in England after this time were those already born in the country or brought into the country from the colonies. Slavery itself was abolished in common law in England in 1824 by a definitive judicial finding that no court could enforce the claim of one person to own another, although this judgement did not have the effect of immediately freeing everyone held in bondage as parliamentary legislation passed that same year made clear enslaved people who had been brought into England from outside the country remained enslaved, and for those who did not fall into that category they would have had to actively prove their case in a court of law.

And finally, the economic configuration of the transatlantic slave trade meant that Britain was simply not a desirable destination to take captives to from West Africa. Slave traders were not just traders in human cargo but in raw materials and commercial goods, as well. Slaving ships would set out from British ports loaded with commercial and industrial goods that could be traded on the West African coast for human beings - very often prisoners of war or convicts from local powers. These people would then be ferried to the New World where they would be sold to local traders or prospective owners in exchange for goods and materials that could sell at profit back in Britain (or for the cash to buy them, or the goods to barter for them in another port). Although there were direct slaving missions that only went between West Africa and the Americas these were primarily a feature of the late period of the transatlantic trade, from the beginning of the 19th century onwards. For most English slave traders this triangular model of exchange was the most logical and the most profitable. Outfitting a slave vessel to make a round trip from England to West Africa was a commercially risky enterprise compared to the almost guaranteed profitability of using the victims of the slave trade as a bargaining commodity for goods that could be sold at high profit in British ports.

So it's essentially an economic rationale - England simply did not need that kind of forced labour in either its industrial or rural economies, which in turn meant it was not particularly viable to try and 'crack open the market' when there was much more obvious profit to be had in participating in the triangular trade. In addition the legal status of chattel slavery was ambiguous more or less from day one which made for an uneasy climate where slavery was concerned, perhaps with the exception of the 1730s - 1770s when the courts were briefly less critical of slavery. Certainly there were enslaved people in England right up until 1833; the first known black female British writer was an enslaved woman who escaped from the people claiming to own in her in 1831 and published an autobiography, and stories like hers motivated the Government to include a clause in the legislation abolishing slavery gradually between 1833 and 1840 that clarified all enslaved people in Britain were immediately emancipated and did not have to wait until 1840. But knowing how many is impossible. 600,000 people in England in 1831 were listed as servants of some kind - English legal tradition considered slavery to be a particular type of indentured servitude even though the terms mean something very different, and so slaves would be captured in this tally - but we have no way of distinguishing enslaved Africans from the total. My educated guess, based on the peak of 10,000 at some point in the 18th century, is no more than around 3,000 people by 1831, all or very nearly all of them domestic servants.