Agree with all the pp who say point out that it was never a loan, it was always a gift and that much of it was for the upgrades to your wedding that they wanted rather than what you wanted, so there is no way that you would have paid £££ for things you didn’t want just so they could impress their friends with how fancy their daughter’s wedding was.
i would also definitely calculate the cost of what you would have had if you stuck to your original plans vs the cost of all the extras and upgraded bits so that you can turn the conversation around from it being about a £10k gift/loan to it being a £3k gift/loan and &7k of added extras that they wanted (or whatever the amount ends up being), so that automatically it reduces the amount that they can hope to extract from you and if they do try to take it further to court for example the fact that you dispute not only the amount but the terms all helps to show the problems with this being a gift or a loan (in your favour).
Also have they given you any money as a present since the wedding? For example, for the birth of your dc, at a birthday or Christmas? Because if so, beyond any small amount that could be deemed a token amount (eg to go out for a meal or to buy yourself a new little necklace or handbag), why would they have given you money if they were expecting you to pay back your wedding money? If they knew they had already agreed a loan with you, surely they would put any gift towards paying that off sooner rather than later?!
Finally - did any of the amount get put into any written accounts for tax purposes - I know that you can gift certain amounts and the allowances are more if you’re a parent gifting a child money for their wedding… I just don’t know the technical rules surrounding that and whether or not it gets declared by one or both parties, or if it’s just for a gift that goes into a bank account or off it applies to paying towards costs associated with the wedding - hopefully someone on here will know - but also hopefully if it does get recorded in your self assessment for example you’ll be able to say that look, you told us it was a gift, we claimed it as a gift, you’ve said you paid it to us as a gift, otherwise we wouldn’t have included it and nor would you. And then that will also keep at least some of it designated as a gift not a loan.
it does sound like they have suddenly realised how much more expensive things have suddenly got over the last couple of years and that if their investments haven’t done as well or if they have the retire early option that they are tempted by along with wanting to speed money doing nice travelling they have decided to change their attitude to you and your wedding (now they have conveniently had the fancy wedding to impress their friends!) and come back to grab money.
definitely don’t roll over and just pay up - they have already spoilt their relationship with you by doing this so don’t be afraid to make them work for any money they demand off you - they are the ones in the wrong, not you!