hall - £120 for
entertainer is £200 for 1h or £290 for 2
bouncy castle hire is £100
I've done several whole class parties and never had an entertainer. They do a surprisingly good job of entertaining each other, throw in a bouncy castle, balloons and a 'disco corner' where you put on a playlist of kid friendly music and they were all happy, red-faced, sweaty like crazy things.
Have you contacted Council leisure centres? The one near me does Sports hall with bouncy castle, some inflatable stuff like sumo suit, giant games and provision of tables/chairs £120 for a 2 hr party and 30 mins either side to set up/close down.
decorations, food, cake, party bags will amount to another £150-200
Think about the space, if it's big will decorations be worth it? We got banners and all sorts the first year but actually when we put them up they looked tiny and inconsequential. The next year we just bought big bags of balloons and blew them up to throw around and paper tablecloths so about £10. Paper plates and cups added another £5.
Food £50 - cheese or jam sandwiches, big box of supermarket brand crisp packets, jammy dodgers/oreos/party rings, mini rolls/fairy cakes, cherry tomatoes/cucumber rounds, blueberries/melon chunks. Jugs of squash or cheap lemonade. Keep it simple, they are not interested in eating and see it as a blocker to having fun. They will sit for the bare minimum of time acceptable and eat very little. If you buy lots of food you will be taking it home or throwing it away.
Party bags £20 - sweet cones, a couple of balloons each (tidying up for you) and a slice of cake. Go to a cash and carry if you have one or a shop that sells the big tubs of pick n mix sweets.
Cake £20 from the supermarket