LHR T3: going from airside to landside
Not sure if this is the most proper board for the following question, but after having lurked here for some time, I believe that the denizens here may be the most knowledgeable ones in helping me.
I am going to arrive at LHR T3 at around 7am on Sunday (from an overnight flight), with my connecting flight leaving at around 6pm, again from T3. This gives me several hours to spend in London, which I intend to do. I would like to have a shower before I head to the city, but, flying in Y, there is no (unpaid) arrivals lounge access for me. However, my status gives me access to airside lounges. Hence I could go airside (passing the security check, but nevermind), go to a lounge, take a shower, get out of the airport, do my things in the city, come back to the airport (and have an afternoon snack in the lounge).
Studying the maps of T3, I cannot figure out how to leave T3 once I'm airside. The FAQs of this forum instruct how to do it in T5 (a much more difficult procedure than in airports that allow arriving and departing passengers to mix), but not in T3. Is it even possible?