This question already has an answer here:
I have US L1 visa on passport that expires in Jan2014. I am planning to visit India in Jan2014-Feb2014. I have my US H1 visa for which I will go to stamping in India. Now, can I travel through London? The visa I have on passport is for L1 that expires in Jan2014, but my return date is in Feb2014 and I will be coming back on H1. I will be transiting through London between 6th Jan 2014 to 12th Jan 2014.
I read few other similar questions but unable to judge whether to have valid US visa for return date or while passing through london.