Food festivals in London
Hampshire has a "food festival" that lasts all of July with different events.
There is a food and produce show in Winchester on 26 July. Winchester is a pretty town, less than an hour on the train from Waterloo. It is free and being incorporated into the Sunday farmer's market which is usually quite good. Not as refined a event as those in London, but free and Winchester is worth a visit.
Do you mean something like the Taste festivals (e.g. Taste of London, which I'm afraid has just finished)? I don't think they're very pricey, if you buy a ticket bundled with crowns.
I'm afraid this is the only one I've heard of. We've been for a few years running, its really good although more crowded every year (this year we went on Thursday instead of on the weekend, which helped a lot).
Keep an eye out for the Thai Food Festival referenced in the attached link:
I've been in years gone by when it was held in Battersea Park and it's a very good day out from both a culinary and cultural point of view. £5 admission I think.
A few years ago I went to a Japanese Food Festival in a small park in Ealing (where there is a large Japanese ex-pat community), not sure if that's still being held, you might want to do some googling. Not as large or as impressive as the Thai event but not bad either and free admission IIRC.