Over the years, I've engaged in a few discussions on this board about going to an airside lounge in T5, then going landside again, and then returning to the lounge before catching a flight.
I have done this numerous times during long layovers in order to go for a run or to go into London.
Before I was taught otherwise on this board, I used to go to a BA customer service desk on the A concourse, and then wait (sometimes a frustratingly long time) for a BA agent to escort me to immigration so I could clear and go landside.
On this board, I subsequently learned that I could simply ride the transport to the C concourse, stay on board, and then ride it back to the A concourse and access immigration just like all other arriving passengers.

So I now have done this several times, but now the hassle is in re-accessing security and getting airside again. When I scan my boarding card at security, it is rejected, and I then have to return to the H1 (visa check) desk, where the agent routinely is baffled by the fact that I was airside and now am landside, and how to fix the problem. On two occasions, the fix required them to change my seat, and then change it back once I was airside.

I take it that this has something to do with the "ready to fly" system. Is that correct? Is there an easy fix to this problem, or should I just expect to deal with this hassle if I want to go to an airside lounge before going landside again?