Any UCAV would either need a fair degree of autonomy or else some very sophisticated control methods. In an era when it is still easier to communicate with the dead than with Cyprus Flight Watch, what degree of assurance could there be that any RAF-operated communications circuit would be sufficiently reliable to control a UCAV? Even then, either someone would probably turn the on/off to off, the cleaner would pull out the plug to plug in a hoover - or Plod would ban the transmission of confidential information....
UAVs are OK for recce in hot areas - and the odd A/G munition has been pooped off by them already - but we're a very long way from sufficiently reliable systems to control anything more than over-the-air re-targetting of recce sensors, I would guess.
Quite right that the only real reason that They are interested in UAVs is to save cost...