B A
I don't recall exactly when they stopped, but live testing of seats is no longer carried out. The variety of test equipment and associated telemetry is a far better indicator of seat performance and generic effects on the fragile user.
M-B have a couple of sites for manufacture/assembly and testing of seats: Langford Lodge in NI has a rocket sledge/rail test track; Chalgrove in Oxon has a number of static test rigs, and they also conduct drop tests from helicopters, and "live" ejections of a dummy from the back seat of their two Meteors (at least they had two when I last carried out a CAA aerodrome inspection in 2008). Again, all these activities are monitored elctronically and filmed from many angles.
The company UK HQ is at Denham, where I think they might also have a Maintenance, Repair and Overhaul facility.
Mister B