I can understand why you would feel distaste at the idea of being "charged" to attend a family meal...
But think of it from the hostess's point of view who has to clean the house, do all the shopping, prepare the food, keep everyone happy while they're there, and then do all the clean up.
I didn't catch how many people would be attending the dinner, but I know as a very frequent host of my family dinners, even if you don't pick fancy wine/top-tier desserts/multiple exotic food courses, oh my God does the cost ever add up!
I didn't catch if you mentioned she wanted ask for £30 per person or per family but either way, that's a deal. You'd be paying a lot more than that at a restaurant, and I'm sorry that you think that she's ripping you off, because you brought a bottle of wine and some fancy crackers and that should be admission enough, but no.
The alternative would be that she does all the buying, and then when you all arrive she takes out all the receipts and divides it by the number of people and then you all pay or share, so her asking in advance, for the money, is at least fair and gives you a heads up.
Sorry, but You're being cheap.