OP you asked if you are being used. There's a way you might find out.
If you don't want to stop sleeping with him for any other reason, stop sleeping with him to test whether he is using you. If he is using you and you stop sleeping with him then he probably won't bother with you any more.
I understand that you have not had a relationship for a long time, but could you really trust him now? If he came to work in London then went home at weekends, how could you be sure that he wasn't leading a double life by telling his wife that he is working in London. Going home and acting the loving husband and father at weekends. Find yourself someone who is in a position to commit to you, someone who is not attached. If he is genuinely not happy in his marriage then he needs to make a decision whether to leave or not but maybe you could let him do that independently.
If you truly do love him, step back and let him decide for himself , with no pressure from you, whether he wants to walk away from his marriage or not.
When I finally found out the man I was seeing (and had fallen for) had a wife (no kids , I think I would have throttled him if they had kids and he had done this), I refused to sleep with him. Naturally I thought he was a liar and a cheat . He still wants to see me, he has begged for my friendship and for me to keep caring for him, but there is no way I am sleeping with him. Didn't issue an ultimatum either, I told him I wasn't going to be intimate with him in any way. Not even a kiss. Nobody should have their cake and eat it. I still saw him as a friend only but it's really painful so I've stopped talking to him completely . I would advise against the staying friends for that reason - it's painful and unless you are resolute, it's too easy to end up giving in.
Some people act like they are in a polyamorous open relationship where they just (conveniently ) forget to agree that with their OH ... probably because they are happy to cheat but don't want their OH cheating on them. Double standards
Affairs usually end badly for all concerned. A lot of people, including yourself can get hurt.