Originally Posted by David-A

When you disembark the aircraft and proceed through the terminal you will have a choice of two routes: 'flight connections' (purple signs) which will keep you airside and out of the UK, or 'arrivals'/'baggage reclaim' (normal yellow/black signs) which will take you into the UK.

If you want to enter the UK, you follow signs for arrivals you will proceed through imigration (passport check, or if you need visa etc), then continue and walk through a customs channel. You can walk through the blue channel as you originated within the European customs and vat area. There is very little difference between walking through a blue and a green channel in practice. We don't routinely stop people in the green channel without reason, and with reason people will still be stopped regardless of what route they take!

Arrivals and departures are totally separate (unless and until you have gone through the flight connections route, in which case you transform yourself from an arriving to a departing passenger), so you simply can not get to a lounge (other than longhaul arrivials lounge for eligbile pax).

I know that is almost what you said, but wanted to clarify it, 'public areas' is a bit of a confusing name for airside departures.

[Oh and attempting to leave luggage in an airport lounge while you leave the airport would be a VERY bad idea generally.]