I don't know what to do about this to be honest. You've got a valid visa which gives you the right to enter as many times as you want, so I'd think there wouldn't be a problem. Perhaps I'm not understanding what your situation is though. If you've got a valid visa to live and stay in the UK, but are in the US right now and going back and forth while your husband immigrates, then I can't see any problem with that whatsoever. As far as letting them know you aren't going to use it anymore, wouldn't you just let it expire, and do nothing about it? That's how I thought that worked. I had a FLR visa, and we immigrated to the US before my renewal time was up. My visa is no longer valid as too much time has passed, but we never told anyone about it. Seems like we would just reapply and do the whole system over again if I wanted to go back to the UK.