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Indian Rupees are a restricted currency so strictly you're not supposed to take them out of India which is why you can't find anyone to change them. The Bank of India has branches in the UK - London, Leicester, Birmingham and Manchester amongst others so they may be able to help but I would have thought the transaction fee on £50 would not make it worthwhile.
I think Rupees are actually not a hard currency, and meant to only be available in India, so I don't think any banks will exchange them. I think you are not meant to take them out of the country at all.