Lonsdale,
NB that the UK MoD already owns three surplus MRA4 airframes.
NB also that BAE have given a good idea of the extra cost of adding 3 R5s to existing MRA4 support arrangements.
The MRA4 has, of course, not been a great advert for BAE. But no worse an advert than the 737 AEW&C or the P-8 have been for Boeing.
My understanding is that you could get one RC-135 into service more quickly than one R5, but that getting all three aircraft on charge would be quicker with the UK solution.
But the key point is that while the -135 is great for Afghanistan, the Nimrod is a better solution overall.