I don't normally read the Agony Aunt sections--honestly!
But a headline in the Times Online caught my eye:
"My husband read my emails and accused me of infidelity"
She is very clear that she has never had an affair. It is actually, quite dangerous to accuse a spouse of adultery especially when it isn't correct. Legally as well as to your ears!
Grounds of divorce do include adultery (which is sexual intercourse with someone of the opposite gender) but if the other does not admit it, how are you going to prove it? The normal proof, certainly when you want to get a UK divorce, is for the other to admit it.
Associating with another might be another ground of divorce which is of unreasonable behaviour.
But, take care, as regularly alleging adultery which you can't prove (and may not have taken place) is very likely to be viewed as unreasonable behaviour and you might end up being divorced for it.