You could try attractiontix for your flex tickets. They email the ticket which you exchange at Universal for real tickets. I am just back from Orlando last month and did not see huge difference in price when I checked the websites.
We stayed at Quality Inn International and like you we where on a budget. We stocked up at the local supermarket and as our room had a fridge and microwave we had scrambled egg for breakfast and took sandwiches, muffins and drinks into the parks.
The hotel had vouchers for Cicis pizza which was across the road, buy 1 pizza buffet at $6 and 2 drinks and get another pizza buffet free. Probably our favorite place, pizza was great and lots of variety.Also Panda Express across the road, Chinese buffet from $6 which was very good.
McDonalds and Burger King close by but I think might be tight on £400 for 5 of you unless the exchange rates improve. If you self cater it might be possible but I think a budget of £10 per day per person would cover it.
You need to budget $14 per day for parking at Universal Orlando or you could walk (about 30 mins from Quality Inn International)
You will have a brilliant time at Universal Orlando and hopefully The Wizarding World of Harry Potter will be open, hope you have a great time :)
You could check this website for more info
http://orlandorollercoaster.com/