Technically, Britain has never had aces, since the concept was deemed to be inappropriate by Trenchard when he was GOC RFC in France...
However, assuming that the recognition of five victories is the criteria, the last British 'ace' would have been in WW2 or Korea. While I know that a couple of the pilots flying F-86s on exchange were already aces, I'm not sure whether any of the other chaps who scored victories raised their tally to five by gaining a kill. (I feel a visit to the college library coming on...)
Since then
AFAIK, the nearest to 'acedom' that any UK pilot has been is 3 victories - a tally shared by Sharkey Ward and Dave Morgan in the Falklands.
<leaves desk for library>