1) Why as big as a Nimrod/Why manned?
see my post at *9
2) Why three when the RAF is "half the size it was when we justified 3 R1s?"
Because it's a capability that is even more 'in demand' in this unstable post Cold War world than it was during the Cold War.
Because it's a capability we need every time we go on ops, and in between - it's not a useful, niche capability that we haven't needed for years and many never need (cf carriers....)
Because it's something we do very, very well, - such that R1s and PR9s have often been the first UK capabilities that coalition commanders have asked for - it is a capability that gives us greater influence in coalition ops.
3) Why not E-3/707 based?
The 707 is now out of production (with the end of the E-3/E-6 lines) so the 737/C-40/P-8 or 767 would be Boeing's offerings - and the A320 or 300/310/330 would be better options.