No.51 Squadron remain rightly tight-lipped about what they do, and how they do it, but I learned a lot about R1 a couple of years ago when I had to write about 51, and was lucky enough to talk to a number of former squadron members.
There's no compelling reason why you couldn't use a smaller airframe (basically with just the flight crew) or a UAV, datalinking the take back to ground stations. Apart from bandwidth and situational awareness and technical issues, that is, which have led the leading exponents to continue to practise Elint using airborne operators, on-scene, in the jet (or turboprop, as the case may be).
Even then there are markedly different philosophies - with some emphasising greater automation and differing levels of inboard interpretation/exploitation, and with 51 stressing manual tuning of receivers at the other end of the spectrum. They can carry up to 28 crew on the R1s - which are extremely cramped - and if we want to continue to have excellence in this area (and the EP-3 and RC-135 blokes seem to rate 51 very highly indeed) then we need to let 51 do things the way they do things, and therefore we need a BIG airframe, and one with widely dispersed, very rigid and stable mountings for some of the antennas. If you're going to triangulate, you don't want your antennas bouncing up and down at the end of a very flexible wing, after all.
And if you want to give them a few Global Hawks as airborne antenna platforms, augmenting the manned assets, I'm sure they'd work out interesting ways of using them!