Hi y'all

I know the UK law prohibits smoking in any enclosed public space. Therefore I can only smoke outside the airport buildings.

Im coming from OSL, will arrive at T5 and got a connection flight to HKG from T3, I do have a 3 hours layover and could need a smoke in between. So my strategy is, after arriving to T5 and the first thing is to catch Heathrow Express to T3 (am I still on airside?) clear immigration, get out to light a smoke and head in again, am I allowed to do that?