The 'Indian' in the phrase 'Indian Restaurant' refers to the Indian sub-continent, rather than to the country. Over 90% of Indian restaurants in the UK are run by families with their roots in Bangladesh, rather than in India.
Prior to independence in 1971, Bangladesh was East Pakistan. Since many of the families who run 'Indian' restaurants came to this country prior to 1971, they were actually Pakistanis when they arrived. So there are really lots of Pakistani/Bangladeshi restaurants in this country and very few Indian ones!
Chris