Look for leisure centres and community centres with sports halls.
We've hired a community centre hall for a Saturday in May, £106 for 3 hrs. They provide tables and chairs and set the room up, we then have to make sure it's in the same state they left it when we finish.
We have a bouncy castle with a slide hired to arrive just before the party starts and be picked up just after it ends, that's £95.
Haven't quite decided if we will be hitting up Costco the day before and buying a load of premade platters of sandwiches or ordering a stack of pizzas to arrive as the party starts, it will be one of those as I already know I can't be bothered to make the food.
We'll take paper tablecloths, plates, cups and bamboo cutlery on the day as well as some banners, blutack, balloons, string, duck tape, bin bags, a battery operated speaker and my phone for music.
The party will be 2 hrs long, we have half an hour either side for set up and break down. The kids will run wild for the first hour, then we'll herd them to the tables to eat a bit, sing happy birthday and eat cake, then they'll run wild till their parents come back to get them.