Just before Peggy left England for America, she did a biographical Radio 2 series with Jim Lloyd. I can't remember the details too clearly, but I recall Peggy saying she'd been deported from Britain and left stranded in Paris, at the same time being pregnant by Ewan.

The upshot was that she couldn't get back into this country without becoming a British citizen, and the only way she could do that was by marrying someone British. AC was also in Paris at the time and agreed to do the job so that she could get back into Britain.

I've never known what Campbell's motives were, although I've heard it said that Theatre Workshop paid him to do it. Personally I prefer to think it was just a humane gesture on his part.