I am so sorry to hear all this.
If your GP would be prepared to prescribe one week of high dose steroid tablets; then you would be able to see if it is the RA. This was done for my daughter by the Registrar as he didn't think it was RA but I did... so he prescribed them... and all the swelling went down!! Her hip had completely seized up and she couldn't walk (she's 24 yrs old). The next week we were due to see the consultant fortunately and she said, as soon as we said how effective the steroids had been, that is was definitely the RA!!! So this would be worth a try seeing as your GP sounds quite helpful.
She does take MTX all the time but does need steroids quite often to keep the RA at bay - but the MTX does definitely help. I am on humira as well as steroids and I need both of these to keep mine under control.
I wouldn't personally recommend stopping the meds (although you don't say what you are on) as this can then cause a really big flare, which would be difficult to control... although in your situation, I guess one could argue, that doing without them.... then go to your next appt having not been on them for 2/3 months - would definitely show if it is the RA. Presumably, though, if you are already on RA drugs - they must be thinking it is RA. It all does sound a bit of a muddle for you.
One other option would be, if you can afford the £150 or so, to see one of the consultant's that you have heard is good privately... and see what they say. The downside would be if they want to order any tests, as these are rather expensive privately... but if you can obtain copies of all your results from your GP you could take these with you.
RA is so frustrating, we can all sympathise with that.... and getting the right treatment can take such a long time. I really do feel for you. You do sound like you have a helpful GP - so maybe have another chat with them about what you could do next.
Hope you can pick something out of the above that might help!
Anne