Ivan,
Not a 707 because the E-Ds won't last that long - and commonality with FSTA will be more useful than commonality with a smaller, shorter lived fleet.
Not a UAV because while it can do Elint, it can't do it as well as an R1, can't do it in the same way, and is limited by bandwidth, etc.
Big because 51 and the EWAU (the experts in this field) feel that big crews, manually tuning receivers and interpreting and exploiting, is the best way of doing the job. I think they have demonstrably shown that they know their onions better than you or I in this field.
In today's world, having this degree of capability is useful for the UK for its own interests, and not just for supporting coalition ops.
It is far more relevant to today's ops (and in the GWAT) than Trident, FRES, Type 45, Brimstone, Meteor, the Household Division, Challenger, or Typhoon.
And much more useful than bloody aircraft carriers, which we haven't needed since '82.
Cutting down from three Elint platforms would be short-sighted and stupid, and there are good reasons to consider doubling the number, and giving them other ISTAR capabilities as well as Elint.