Premium check-in is in Zone A, Fast Track is available at security and usually pretty quick.
After security, turn left through the departure lounge and follow the signs for UK/Ireland departures. You'll go through the photo ID check (no Fast Track here).
There is only one lounge - the BA UK and Ireland lounge (the former BD London Room), located by Gate 8. Nothing special by any means, but adequate. Definitely no CCR that you'll be used to
There will be Fast Track boarding signed like T5.
Once you're in the domestic/Irish area, there are limited shops, so do any shopping you want before heading to the lounge through the photo check.