If you have breakfast at your hotel, B&B or hostel, I wouldn't expect to spend that. You will have plenty for budget eating with splurges.
IMO Steak & Kidney pie is the best pub grub. It really doesn't taste like kidney, more like a really good version of our beef pot pie.
Fish & Chips wasn't great. Although I ate a lot of it, the Brits seem to like it very greasy. Maybe if you let it drain?
The Brits serve peas with almost everything.
If you like hot mustard choose the English mustard. The French mustard is basically what French's mustard is here.
The only polite thing to do when drinking is to ask for a pint of their local brew. Every area is proud of their local ale/bitters/stout and you'll start off on the right foot every time.