You will arrive into the domestic arrivals at T3, ground floor - this is segregated and does not mix with departures.
Actually if you follow Flight Connections signs you can nip upstairs into the departure lounge, there are no checks as you are UK security cleared. Just turn right towards Gates 144 and the like and head upstairs. I have done this on occasion to use the shops before heading out into the big old world.
If you have a boarding card, there is no need to exit and reclear security.