So from another thread, I'm getting that LHR T3 (and maybe all terminals) have physically segregated departures and arrivals area, and there is no way to cross between them without reclearing security? Is that true?

I'm now even concerned if I can use the No. 1 lounge at T3 on arrival. I know that LHR has this weird thing of designating departure vs arrival lounge, but I'm looking at the angle of is it feasible to get to the No. 1 lounge at T3 on arrival, as opposed to the intended/recommended use.