Cambsnan
How big is the holiday? Maybe book a big house in the UK for a long weekend and invite them to come for as long or as short a time as they like? could you invite her folks as well?
And if they can't make it, or don't want to come? Does the DIL get no say on where and with whom she spends her time off? She's already had offensive and sexist insults thrown at her (not by you, Cambsnan), is assumed to be a harridan, and more, because the family has not asked the OP to join them on holiday. Manipulating her into going whether she likes it or not is unlikely to help, IMO.
MoaningTurtle, how do you know your son is upset by the arrangements? Have you been discussing your DIL with him behind her back? I would see that as a huge betrayal in her shoes, and if anything would put me off seeing my MIL as part of my close family it would be that sort of behaviour.