Your husband is disrespecting you and your wishes, and your agreement. He is being a selfish idiot. He probably listed this woman as his sister, so you wouldn't feel bad if you saw it was 'his girlfriend,' 'wife' or 'significant other.'
For your husband to not want to hurt her feelings - but has no conscious when it comes to hurting you - what does that say ? It says he doesn't care about you. I know guys like this, it disgusts me. Personally, that is why I am divorced, and will never by choice re-marry - I don't see ANY man that can be truly 110% faithful to his wife - not just physically, but emotionally. It seems that after some time, men all want to stray and find other women - and being this woman is in England this is a 'fantasy woman' for him - he can say to you, 'I am not cheating on you,' yet emotionally he is. He is also disrespectful to you... You know he will not willingly 'end this' personally I say NO more Facebook at all - delete the account permanently, and make sure he is fulfilling on it. If he is not, can't help himself and does it thereafter you have the option of counseling or divorce.