It doesn't really matter which university you choose so long as when it comes to your selection process you can show that you kept on top of your academics, were active in sports teams and showed elements of initiative and leadership (i.e club/society committee positions, leading expeditions etc)
As far as BEng/MEng goes, either is fine, obviously 4 year courses gets you the seniority, that bit more life experience before you join and a step closer to getting chartered but you can do a Masters while in the RAF as well.
It would also help if the university you chose had a strong UAS attached to it.
Out of interest, are you considering AS or CE?