Originally posted by Tango:
If you have more than 2 hours, get out and get some fresh air. Under 2 hours you won't have time.
I'm of the view that the air inside LHR is cleaner than the air outside. It's always choking with diesel fumes around the arrivals area. A characteristic London smell, like hops in Edinburgh.
There's plenty to do airside at LHR, particularly if you're a shopper. A number of new restaurants have recently opened in terminal 3, not sure about the others as I always fly UA. When in London, I prefer LCY due to its proximity to the city (and my house!) and 10-minute check-in.
Regards,
Matt