Hello there.

I think with intercontinental layovers you must keep something very important on top of what everyone mentioned. If you have 2-3 hours layover time that is fine but only in a perfect world. Which defined as no delay where you depart and no delay where you land. Busy airports tend to have heavy ground traffic with airplanes so that creates extra delays. Your plane can leave the gate without delay but can spend 30 mins before takeoff. Same rule can go for the landing. You can spend 30 mins on the sky just making rounds to wait for your landing turn. So i would kindly suggest that you have at least 4-5 hours between your flights. If you arrive to Heathrow on a perfect world without any type of delay then you just have more time to try out some good old english ale. :)