We are from NYC, where we have some pretty good ethnic food (not so much Mexican though...more "chain restaurant" type 'Mexican'). DH and I love a nice variety of food with good and unique flavors. We also enjoy a good burger, but once in a while...not all the time.
We really loved San Angel Inn (Mexico at Epcot) on our last WDW trip. We ate there for lunch and we found the food, atmosphere and service all to be excellent. We will be returning on our July trip once again for lunch.
We also loved Kouzzina by Cat Cora (Boardwalk, short boat ride from Epcot). The food is Mediterranean (a lot of Greek influence), and we thought it was delicious.
Sanna at AKV was another gem. It is not authentic Indian food, so it wouldn't be what you are used to in the UK, but the flavors are unique and delicious, the restaurant is beautiful and relaxing, and it is wonderful to watch the animals out on the savannah while you eat.
Biergarten (Germany at Epcot) is our favorite WDW restaurant. It is a buffet, with live entertainment and a dance floor (not too many people on the dance floor, but the kids enjoyed it). My grandfather was German, and he was extremely picky with German food. When I brought my mom to Biergarten in 2010, she said that he would have approved of and loved that place. I knew that already though because there was only one German restaurant in NY he would go to, and he always got sauerbraten (his mother used to make this and he was very picky about it). The sauerbraten at Biergarten was just like the German restaurant I went to with him as a kid! They do have more "American" offerings as well (like mac & cheese for the kids), but these could easily be avoided and you can have a terrific meal with tons of variety. They also have a wonderful beer sampler.
We also really liked Chefs de France. I had quiche there for lunch last time we were at WDW and it was just as good as the quiche I remember having in Paris 15 years ago. The baguette bread was just as good as it was in the real France too.
I haven't eaten at the Italy, Japan or China pavilions (and don't intend to) because we can get excellent Italian food and Asian food here at home...we like to have food we can't get here when we are on vacation.