There are probably several hundred Indian restaurants in both of those cities, scattered throughout. It's actually fairly hard to eat bad Indian food in the UK. Sure - there are areas with concentrations of Indian restaurants (e.g. Brick Lane or Drummond St in London) but you absolutely don't have to go there to get good food. I will give a quick shout out to Southall though (short train wide from Paddington) due to its Indian population and hence availability of particularly cheap and good quality food.