I think if Ramblingrose has never entertained the thought that there may be ulterior motives for her husband spending so much time in UK and excluding her it probably has been quite a shock. Nevertheless I too am of the opinion that there is more to this story especially as it has been going on for 3 years now, one way or another.
Is there any way you can speak with your in-laws RR? I suppose it depends on how you all get on but I think I would say something like "Hi, am coming over next week as a surprise for DH, is it o.k. for me to stay with you?"
See what the response is?
OR you could just turn up? "Surprise"
Either way, I would have to know exactly why my husband seemed to prefer living apart from me (which is technically what he is choosing to do?) There is one thing being private and wanting to go to appointments alone but it is another when you don't want to spend every other minute you can with your partner. Also, from my experience of NHS, it can be weeks from an initial appointment to follow up and subsequent follow up appointments? You say it isn't worth him returning home for a few days?
As everyone else says - not wishing to upset you in any way at all - but something just doesn't add up correctly. Do you have any other relatives to help you out if in-laws are a no no? Good luck.