Here are a few threats completely off the top of my head:
Russia - not much capability at the moment except all those nukes but a willingness to use what capability it has pretty aggressively (vide Georgia) where will they be in 30-40 years time?
China - busy securing the World's resources on the open market but will they defend their position by force in due course?
Iran - where is that going?
Collapse of the eurozone following a Sovereign Debt crisis and a return to European nationalism?
Regional conflicts which suck us in?
India/Pakistan
Iran/Israel/Syria - whole Middle East?
Russia/Ukraine/Georgia
Various African conflicts
China/South China Sea/Japan/Koreas/Phillipines/Vietnam
and then
AQ sponsored global insurgency
Irish republicanism - still not gone away.
and finally the Argentinians still want the FI.
Actually come to think of it, there is no better time to make massive cuts in defence.