My question though relates to the right of the security guard to become involved in what is in effect an airline and immigration issue.
G4S at LHR and FRA are employed by the airline to do exactly that. I was not alallowed into the gate area for FRA-LHR without a rather full check of my right to enter the UK. The right to enter the US also means they're entitled to ask you for proof you intend to leave.
Why not help the person do their job next time by carrying the details or is your beef that a mere pleb asked you instead of a senior customs officer or airline employee?