Yes I take the point about the commuter belt, but to put it bluntly, people live there if they can’t afford to live closer in or they want more space for their money. Beggars can’t be choosers, sadly. The fact is, OP is the main earner here. She is the commuter. So she gets to call the shots, I’m afraid.
Eh, no. She can decide she isn't moving somewhere but she can't unilaterally decide they're both going/staying anywhere.
I agree with you that some kind of compromise option is sensible (although DP doesn't seem to, which would concern me) but this seems much less suitable for what either of them want than Cardiff or perhaps Bristol would be. No buzz, no family, not really any more to be had on their budget.
Had a quick look on rightmove as I've only been to Caterham once, and that was the dearer part over 10 years ago. Most of what's available for the 280k someone reckoned they could borrow upthread is still flats, and not that many of them if you're under 60 and not a cash buyer. If anything it would make more sense to be in Croydon for that price, since one of them would actually like it there.