Not forgetting that you need to have the FAA pitot static and transponder checks every 24 months. These can only be done at an FAA repair station in the UK (of which there are quite a few, but your IA / A&P can't do them).
Won't a GTN650 (which I presume in an IFR certified GPS) give you Nav as well, so you'd have VOR. If you are not going to fly an NDB approach then you could forget ADF (as you would be navigating enroute with your 650). Many +DME approaches specify that they can be flown by getting radar ranges from the controller.
Personally I think that an IFR certified GPS should easily be able to do away with the requirement for DME, I dunno why it takes so long in the UK though, it went donkeys years ago in the USA.