on the other hand the slave trade was banned in England by 1100

The idea that William I banned slavery is one of those interesting little ideas that is continually batted about without really being true. We actually have very little evidence of any legal code at all dating from the reign of William I, beyond two surviving charters, which make reference merely to the maintenance of laws and rights 'as in the time of King Edward.'Given England's well established tradition of extensive legal codices, William and his immediate successors appear to have been happy to keep the majority of English law based on precedent, changing individual laws here and there, before a new legal framework appeared properly in the mid-12th Century.

The idea of the Guillaumian abolition seems to come from the Leis Willelme, a codex purportedly of the "laws of William I" but which has actually been dated to approximately 1120-1150, some 30-60 years after William I died. The reign of Henry I saw the publication of a number of putatively 'eleventh century' legal codices including the supposed law codes of Cnut, Edward the Confessor and William I, as legal scholars attempted to establish a corpus of precendence for new Common Law, based largely on what they thought the law had or should have been. The clause in the Leis Willelme that's supposedly a ban on slavery also actually isn't an act of abolition at all. It specifically is a ban on sellin English slaves outside of the country, which wasn't a new law but was actually simply a reiteration of clause which first appears in the 7th Century legal code of Ine of Wessex some 400 years earlier, forbidding the sale of an English penal slave overseas. This Saxon law was a direct contrast with preceding Kentish law codes which suggest that a thief caught in the act should, in fact, be sold out of the country into slavery. The supposed legal basis for 11th Century abolition is then, rather, a 12th Century construction of an idealised 11th Century legal codex including a 7th Century clause essentially creating a legal difference between British slaves and English penal slaves.

Despite this, slavery in England does indeed seem to have rapidly declined at some point between the compilation of Domesday Book in the 1080s (when slaves still comprised ~10% of listed households) and the writing of William of Malmesbury's Gesta Regum Anglorum and Gesta Pontificum Anglorum in the 1120s and 1130s. According to William of Malmesbury, this rapid decline was to a great extent attributable to a moralist abolitionist movement, personified in particular to Bishop Wulfstan of Worcester. In his Vita Wulfstani, William wrote:

A detestable, evil deed; a deplorable, shameful act; not even bestial men, mindful of affection, betray their own relatives to servitude, following their own blood. So Wulfstan, just as I declared, gradually destroyed this long-standing custom, handed down from remote ancestors to their descendants. For knowing their stubbornness [would not be] swayed easily he stayed near them often for two or three months, approaching them every Sunday, and scattering the seeds of divine preaching. And truly he grew strong among them over time so that they not only rejected the error, but set an example for others throughout England to do the same. Indeed, one of their number, who stubbornly resisted the commands of the bishop, was soon expelled from the village and deprived of his eyes. In that affair I praise their zeal, but condemn the act, although once the spirits of wild men have been encouraged, no one is able to oppose them with the force of reason.