Leicester is rated the native curry capital, not over influenced by 'interpretations'. However the best curry place I know is the Taj Mahal in Penzance. None of these are 'posh', just serving superb food.
Other nationalities specialities I don't really know about, but they certainly exist outside Maggie's Girdle (the M25). When I used to travel for work I used a lot of different eateries, and really didn't have a bad experience. However, I'm not hyper critical.
Nowadays it's just me, so I'm not out much, preferring my own company.