Originally Posted by
lon_flyer
I second the idea of jumping on the tube/train into London. You'll get a return trip, decent meal, couple of drinks, and an hour or two of sightseeing for less than you'll pay to get into one of the (sub-par) third party lounges. Six hours is more than enough time. If you have a US passport then you can use the e-gates, so unless there's a problem you won't experience any delay passing through passport control.
Tube is cheapest but slowest (45-60 mins)
Heathrow Express is quickest but expensive (15-20 mins)
TfL rail is the in-between option (30-ish mins)
The problem I found with TFL rail (first time use a couple of months ago) were the larger gaps between trains and also that they only went to T5 at the weekends (not sure if that has changed) necessitating a transfer. Also, we were held up at signals so it took an age to get into Paddington - negating time saved (was also delayed on way out). Almost made me wish I'd taken the tube - which at least is more frequent and cheap. I've been back to the HEX after that simply because you can't time when you're going to be on the T5 platform when coming back.
Definitely a vote for London though - 6 hours is more than enough.