My goodness, I'm gobsmacked and speechless, which is rare for me!
Like the others say, it is normal for the hosts to provide the meat. We've provided the drinks and puddings before.
IMHO, can they really be 'friends' because if they knew you well enough, then they'd know that your food budget is restricted as a necessity. After all, that's almost half of your food budget: I as a friend wouldn't impose that on anybody knowing that. Not in a million years.
I would be inclined to mention casually sometime rather soon before they forget the BBQ request, that your weekly food budget is £60 because that is all you can afford, even with cutbacks, etc. Hopefully they'll get the hint not to do it again.
My DMIL used to insist inorganic this, that and everything. We couldn't afford it. I couldn't grow it for multiple valid reasons. I
put things in plain bags and produced them like that. Funnily enough,she deemed them 'delicious', much to my own private amusement. It was obviously psychological for her!
I would be inclined to make your own burgers next time or buy cheap and take in plain bags. Bluff it. If they don't like it, then next time they will provide the meat! Or maybe I'd suggest that next time they provide the meat and you provide salad and a pudding of ice cream!
Buying cheap meat, but 'well marinated', also hides it's origin too. A there's loads of reasonably priced marinades, herb mixes, etc, that means you have to remove the original packaging which goes without saying, hahaha... And yes, I've used that tactic too. Works a treat... 😉😂