I may be way off line here, but I'm just getting this image of the sort of Mother of the Bride from hell who lives for the day her daughter gets married - and it's as much about being a day for her, as for the couple getting married. Maybe others know "the sort" ? ..... my ex-MIL was one .... mutton dressed as lamb, centre of attention ............. oops, getting carried away with myself.
The PILs being separated could in MIL's eyes be "justification" for her request - but only, if you are incredibly insensitive to other people's feelings, and also believe that your daughter is "entitled" to a certain standard of wedding. Going back to the MOTTBFH idea - many of these women look upon the occasion as an opportunity to "pay back" past invites to various of their friends and acquaintances too, a chance to be the "gracious hostess".
That still doesn't excuse her approaching you, nor her sneering at your lifestyle. You are perfectly entitled to eat out as and when you wish without it having to be seen as some indicator of your wealth (or not). And what the heck that's got to do with subbing a couple who must have at least £2k disposable income each month (after rent, bills, travel to work) I don't know.
If they in particular can't afford it, then they only have themselves to blame. Why should their inability to save, cut back, take a loan or Godf forbid wait entitle them to help from people less well off than them ?
I know this is really irrelevant to your situation but I've been engaged 2.5 years. We can literally not afford to get married. Yes, really we can't ..... even on a "shoestring". Right now, we couldn't even afford the registry office fee, before you consider it might be nice to be dressed in something other than jeans. If we ever do somehow manage to scrape some extra money together, there are 101 things we need to do to our house which really should take priority. So, we wait ......... ad infinitum, probably. Both my potential MIL and BIL are well off but we wouldn't dream of asking them to pay for our wedding.
I really do think that family or not, this whole thing is incredibly rude, and not only that, but greedy and shameless, given that the couple seem to completely capable of paying for themselves. Which other family members have been approached ..... how do they feel ?