We are flying from SAT to IAD next Saturday on United and then again United from IAD to LHR. We have a long layover in LHR and will be waiting in the Virgin Atlantic Clubhouse before our flight to CPT. Will we have to clear security in LHR since we will be in transit? I just thought it would be nice to keep a water bottle with us. I am not sure of the procedure to follow.

Fran