Question about LHR fast track
Hey everyone. I have a question for the experts about LHR (I haven't been there in years). I will be flying EWR-LHR-BKK (F on UA and J on TG). I have quite a long layover in LHR so I will be going out to explore the city. When I return to LHR from my adventures, what is required to gain access to fast track for security/immigration, etc? Is the TG boarding pass sufficient or do I need an invite (in which case would I get this from UA when I check in at EWR even though I'm continuing with TG, or can I get one at the arrivals lounge before I head into the city?). I tried to do some research online and it seems that a boarding pass in J should be sufficient, but I just wanted to confirm this. Thanks for your help!