Originally Posted by
Genius1
You won't go through customs, just immigration. Flight Connections UK & Ireland immigration usually isn't too busy.
To be over pedantic, you do go "through" customs, it's a red phone on the wall at the back of UK Flight Connections, and that's it.
UK passports can take 15 minutes at that hour, or if you have e-passport access that would be a reason to go landside and then up to South security, there are no e-gates for UK Flight Connections.
Either way I'd reckon you should have up to 2 hours in the lounge all being well. The wait time for showers in the North lounge is usually shorter than South.