Yes it happens up and down the country as you describe, at about £100-200 for a family of 6. Reasonable turkey, 2 types of stuffing, 2-3 veg, 2-3 types of potatoes, a starter, a christmas pud & another dessert, maybe a couple of bottles of prosecco. Add in a few nuts, crisps & chocolate, even add in the extra costs of heating, lighting & cooking, and you're still well within that amount.
And the OP wasn't talking about helping with the washing up etc, she was talking about doing half of the shopping, prep, serving, clearing away and washing up.
There's a big difference between "hosting" and providing the venue as your house is the most convenient. My father was one of 12 so had a massive family. We had the only house big enough to fit everyone for all the major family event - big birthdays, engagements, weddings, funerals etc up to 80-100 people every time sometimes 2-3 in a year. As our house was the venue, did that mean we were the "hosts"? In your mind that would mean we would be happy to pay the many hundreds of pounds every time for alcohol alone for these events. No, the person/people celebrating would pay for consumables. When it was one of our own celebrations e.g. my DSis's wedding, then we paid, but people also helped & brought stuff like alcohol, food, nibbles etc.
Hosting an event can mean different things a) depending on circumstances, b) according to the abilities of people to pay and c) your own upbringing and experiences.