Having just read the benefits bashing thread, £816 sounds like a really healthy sum to get by on, despite your expenses.
As others have said, you can make your own pizzas or even get the children on board to make them with you.
The present for your DS's friend doesn't have to be £15. The Works do book bundles for a tenner. Maybe get one of those, divvy it up and it will be two or three gifts going forward.
When my DC were still living at home 7yrs ago, I didn't have that much left a month. I was working three jobs but managed, although I appreciate costs have gone up since.
As for Disney, there are torrent sites where you can stream Disney, Netflix, Apple, Prime, latest cinema releases for free. I've used them for years.
Other than that, keep on keeping on! You're doing great. Have a fabulous time on your DC's birthday.