I hope this answer meets the rather exacting standards of /r/AskHistorians. Of course it is difficult to establish why something didn't happen, but perhaps this answer will, at least, provide a general overview of some of the main reasons why Britain did not have a mid-nineteenth century revolution.

I think the standard, accepted narrative goes something like this:

Firstly, Britain had had an ideologically driven and violent revolution - in different forms in England and Wales, Scotland and Ireland - in the 17th century. The Restoration Settlement after 1660, and the formation of the British State in the period from 1689 to 1714, an attempt to bring that revolution to a close and to enable the elite - the financiers, the merchants, the gentry and the aristocracy - to close ranks. Having curbed the power of the Crown, at at the same time fought off radical advances from the democratically-minded Levellers and radicals, this ruling elite held power both in national and local politics. They exhibited a profound hostility to abstract ideological thinking, in favour of practical, pragmatic, historically-rooted institutions. Crucially, those institutions included a powerful (if horrendously corrupt and unrepresentative) Parliament, as well as an independent judiciary, largely autonomous (if oligarchic) local institutions, and freedom of speech and religion (within limits).

Secondly, the French revolution terrified the British elite: they saw the violence of the terror and the levelling effects of the Jacobins, and they wanted none of it. There was a massive propaganda effort in support of the existing (unwritten) 'constitution', as well as quite a lot of self-congratulation from writers from Edmund Burke to Walter Bagehot.

Thirdly, this meant that the liberal institutions that middle class revolutionaries wanted in continental Europe already existed. The middle class had more to gain by working to reform and improve existing institutions, and more to lose by siding with the workers to overthrow them. The middle class looked upwards to the upper class, whom they sought to emulate and - if they got rich enough - to marry into, rather than associating downwards to the poor. This was especially so after the electoral reforms of 1832 and 1867, which enfranchised the middle class.

Two other factors were: (i) the role of methodism, which while it had a strong social conscience and did much to promote reform, was an anti-revolutionary religious movement, which channeled much of the enthusiasm of the skilled working and petit bourgeois classes away from politics and into religion; and (iii) the role of the empire, which was a massive safety value enabling emigration for those dissatisfied with conditions at home.

To this, however, I think two caveats should be added: firstly, the UK did come quite close to revolution in the 19th century - strikes and Chartist activity, were seen to be a real threat to the ruling establishment, which was repeatedly put down by force (most notably in the Peterloo Massacre) and by selective repression. Secondly, one could argue that there was, in a way, a revolution in the 19th century - it was just a slow and peaceful one, rather than a sudden and violent one. Not only was the franchise steadily expanded and local government placed on an elective basis, but major institutional reforms created more of a level playing field - the Cardwell Reforms in the Army and the Northcote-Trevelyan Reforms in the Civil Service removed a lot of the corrupt and privilege that had characterised the British state in the 18th century and laid the foundations for an active, effective state in the 20th. By 1911, all but one of the Chartist demands, which were revolutionary at the time, were met.