Originally Posted by
ducky86
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?
You need to clear immigration before catching Heathrow Express.
But as long as you do not need a visa to enter the UK, I see no problem with clearing immigration, catching HEX to T3, going outside and smoking, and then get airside.