An apology has already been issued (as noted in a sibling comment). That apology, though addressed specifically to the case of Alan Turing, was general, and included "many thousands of other gay men who were convicted, as he was convicted, under homophobic laws...". That is a meaningful statement.

And there are classes of pardons and other actions that could "make it didn't happen", but that would absolve Britain (or, more properly in this case, England and Wales) of responsibility for historical wrongs (and they were hardly unique in that). It did happen, and it would be better to note that it did happen, but that things have changed in such a way in the interim that we, collectively, no longer believe that what happened is acceptable.

http://m.guardian.co.uk/world/2009/sep/11/pm-apology-to-alan...

Ironic that we now have Sir Ian McKellen and Sir Elton John, two men who would have met the same fate in Turing's time.