To be a bit more specific:
ctokelly is correct to say that there is no mandatory requirement age, as this was indeed phased out, but from the UK Government website:
[S]ome employers can set a compulsory retirement age if they can clearly justify it.
In practice, this seems to vary from university to university. Oxford, for example, do set a compulsory retirement age for their staff. From their website:
Council has agreed to maintain a retirement age for university academic and academic-related staff[...] All existing members of academic and related staff who have a normal retirement date of 30 September immediately preceding the 66th birthday will be deemed, from 1 October 2011, to have a retirement date of 30 September preceding the 68th birthday, which shall be the EJRA.
Cambridge have a similar policy: see here and here.
Others, such as the University of London and York, seem not to.
In the other direction, UCL suggest that their employees may need to work until 65 to access their full pension. From their website:
Effective from 1 October 2011, there is no compulsory retirement age at UCL. Staff may voluntarily retire at a time of their choice, subject to providing appropriate notice. Many staff may continue to retire at their pension age (currently 65 years) because this is when they can access their full pension benefits although they have the choice to work beyond this age. Staff may be able to flexibly retire or retire before 65 years (normally accessing reduced pension benefits) depending on their pension scheme rules. (emphasis mine)
The answer seems to be, then, that it depends on the individual university. Oxford and Cambridge do seem to be in the minority in enforcing a mandatory retirement age; most universities stress in their policies that the age of retirement is a choice.
As to when academics actually do retire: I couldn't find any hard figures on this, although the general consensus for the UK across all job sectors is that people are choosing to work longer. I should imagine, however, that the default is still somewhere between the former mandatory age of 65, and the 67 or 68 set by Oxford and Cambridge.