This question already has an answer here:
I hold a valid Indian passport and I would be travelling to the USA (from India) with a valid US Visa the coming weekend. I have a long layover of 10 hours at the LHR terminal 5. I would like to roam around London a bit for few hours using the underground tube. My question is if I need a transit visa for going out of the airport and getting back again. Needless to say I would be passing through the customs and immigration of UK border control.
Please respond ASAP.