Something to think about:
With JSF replacing Harrier and Tornado GR4 and the RAF committed to a 2 aircraft type fast jet fleet (Typhoon and F-35 by the 2020s) what do you replace Typhoon with in about 10 - 15 yrs time?
Whilst UCAVs look great for deep strike missions they really aren't a good shape for Air - Air combat. I don't see anyone friendly developing a new, manned fighter and 10 yrs from concept to IOC seems impossible these days.
Anyone want to bet 138 F-35 (prob mix of A & B) will be the only manned aircraft the RAF will have in the not too distant future?