Hi Grace, So they were on the point of "assumptions" while living together. You said He took a woman into his bed whilst she is there in the next room. So therefore, they are in a "relationship" (not married) but sleeping in different beds/rooms. In this case the woman is at a loss.
She should have thought of this in the first place before living with him. He being a matured man should have done the right thing by talking to her about his feelings that the "assumed" relationship is heading nowhere. For sure he did it to end the relationship, which is really hurting. Perhaps he doesn't have the guts of telling it straight to her face that he doesn't have the feelings of love just lust hence, bringing another woman into his house.
Experiencing this kind of rubbish actions is less hurting if one is in an "assumption" relationship than in marriage because in marriage there is the "commitment".
I guess it's not about British. It's about the person. We all don't have control over our future but we are all hoping that everything will last. For sure every person who had their 1st, 2nd or maybe 3rd relationship thought that it will last forever.