Thank you both very much. I would appreciate clarification on one more question related to Concorde room access. I will be traveling twice through LHR in the next few weeks, each time flying in F from IAH to LHR and connecting in LHR in J towards another destination. But the circumstances are somewhat different after that. I will describe my two trips below and would like to kindly ask if I will have access to the Concorde Lounge each time and, if not, what other options would be available:

Trip 1:

IAH to LHR on BA in F arriving T5
LHR to MAN on BA in J, separate ticket, departing T5
No checked bags; will have to exit customs & passport control area and re-enter post-security (airside) as if I my trip originated locally, in London.
After re-entering the post-security area, would I have access to the Concord Room? If not, what other lounges are accessible?

Trip 2:

IAH to LHR on BA on F, arriving T5
LHR to DUS on BA on J, same ticket, departing T5
Checked bags
I understand that in this case I *will* have access to the Concord Room but I first have to clear security. Is this correct?

I also have a separate question regarding trip 2, and it has to do with where to clear security. Which of these options would you recommend?

2a. Follow the LHR transfer signs and clear security with the transfer passengers, where fast track is usually *not available* (Security lines are notoriously long at the transfer point)
2b. Go through customs and passport control (US Passport), officially enter the UK, revert back, and go through the fast-track security check available to passengers who originate in London.

Thank you all very much for your help with this!