Crab, so presumably Cardiff Radar would also pick up the other aircraft and provide you with information / separation iaw the service you normally request, especially if the other aircraft was using a transponder.
Is your Seaking TCAS equipped, btw? Many modern IFR helicopters are; I'm not sure if the MOD have got around to bringing yours up to date yet like they have seen fit to do with other RAF types. If not, I would think there would be a good case to do so, even in these cash-strapped days.
Seems to me you are presuming that pilots here carry out their own business in IMC "blind", i.e. with no rt contact and no radar service. I certainly wouldn't if a service was available; I don't think that anyone else with common sense would.
Having said that, I did once see a mode 'A' only R-44 (as an unstabilised single, presumably non-IFR equipped and certainly not allowed to fly IFR in UK) who passed only just beneath me in solid cloud under the London TMA a few years back. We were in receipt of a radar information service at the time, he was certainly not on the local radar unit's frequency. The cloudbase was about 1200' and we were at 2400', so he certainly wasn't scud running!