£20 sounds like a great deal, you come in..pay your £20, eat like a king, drink like an alchie and stay toasty warm at someone's expense and you get to leave without doing anything.
The things you don't have to do for this £20...
Shopping for everyone's dietary requirements. Just doing a normal Christmas food shop is a headache as it gets closer to the time, but add in a celeiac, vegan, vegetarian, someone with diverticulitis and so on..makes it 10x worse.
Prepping all the food, depends how many she's doing it for and how she will split the work load, but a Christmas dinner for more than your average sized family can be hard going.
Making sure the guests are happy. Will they have enough food, will it be cooked to their liking. Did she get auntie Sharon's favourite fucking drink they drank back in the 50's, uncle simons lager that no one else will drink!
It all adds up, £20 is literally 1 chefs wage, for 1hour. She will be doing a big job, and it will take her longer than an hour.