If you want a small wedding in the UK, then now would be a good time to 'compromise' and get that.
I'd go for a small weekend away somewhere (Cornwall? Lake District? Somewhere picturesque, I'm sure some other MNers could find you somewhere suitable) and say that you're still doing the eloping idea, but will take both sets of parents, your DCs and siblings from both sides (assuming your DP has siblings). No other guests, no other family. You will have to keep it to that, because once you start asking other people, your MIL will start with her list of 80 again. IME it tends to be all or nothing with weddings, it's often very hard to do a wedding for 40-50 unless you both have very small families.
You could go for a very nice, posh hotel for a fraction of the budget of Vegas, and still be able to afford an expensive honeymoon.
however, if your heart is set on Vegas (rather than just running away to Vegas because you were about to spend a fortune on a day you didn't want), then go.
One other thing though, you say you are paying because your DP has been made redundant, you are marrying the man, you have DCs together, perhaps start having joint savings rather than yours and his? It's all going to be "assets of the family" once you're married, I'd hate if DH was the only one who got to make the decisions about big financial outlays just because he's the one who earns the bulk of the money.