If you want to use the AC on a T5-T5 connection at LHR, do NOT clear security at T5, instead follow the signs for flight connections for the bus to T3. Clear security at T3, visit the AC. When you are ready to head to T5, then you'll need to find the entrance to Flight Connections at T3, which I believe is near where you exited security at T3. From flight connections take the bus to T5, clear conformance, clear security, and then proceed. Make sure to leave the lounge at least 1.5 hours before your departure from T5 as you must clear conformance at T5 no later than 35 minutes before your scheduled departure.

So you have to take the flight connections bus twice, clear security twice, and make it back no later than 35 minutes before flight time, so if you are going to do this you need an extra long connection. And even then I'm not sure it's worth it to visit an AC.