You don't need to do your IR on a twin. Vantage Aviation do it on a single. However, since you have no twin time it would probably be a good idea to do the IR on a twin to build that part of your CV.
I think in terms of investment Vs return, you're unlikely to get one in a short term time frame from doing an FAA to CAA convex, twin rating and IR. I know a guy in Babcock who has converted an FAA CPL IR but I think he had a big wedge of twin time already (he was employed directly from his performance during the conversion - something to think about in terms of where you do your conversion).
EASA has decided to extend Single Pilot HEMS to 65 so I think you can reasonably expect the CAA to do the same already doing it just for HEMS is likely to be questioned