Not As Easy As Adding Kit to a New Airframe
I continue to be amused by the idea that the C-135 airframe is outdated because it is 40+years old and that it is easy to transfer a comprehensive SIGINT package to any new jet.
All C-135s had new wing skins in the 80s. The RCs have a new, specific glass cockpit different from the KC-135 PACER CRAG, new engines, entirely new mission equipment and systems (cooling, lighting, power, distribution in the backend. These are not 40+ year old airframes in most all respects. Every three years, or so, L3 does an upgrade on each airframe, stripping out most of the wiring, backend equipment. The entire airframe is gone over for corrosion (especially under the toilet area in the aft end) and stress. Voila! A totally upgraded airframe with the latest Block equipment. The RAF Rivet Joints will be Block 11 aircraft, the latest coming off the L3 line. (BTW, all RJs are never all the same Block as they are continually changing the backend capability).
Now to just converting some Airbus version to a SIGINT aircraft. Working on emission control and interference, while building equipment to detect, locate and copy low-power signals is not something you do overnight. Working with the same airframes since the early 70s has a tremendous advantage over shifting to a new aircraft. You really must understand some basics about airborne signals collection in today's RF environment to see how absurdly difficult and costly it is to switch to a new airframe, especially for a contract for three aircraft. (Same applies to moving the Nimrod R.1 mission to a new aircraft).
The RJ is the best at what it does because of the history of long development by a single office and contractor (albeit with different owners over the years). Besides, a switch to Airbus, with the fly-by-wire controls, would be a whole new set of problems inside the backend.
RAF has made the best, most cost-effective and mission effective choice.