The loads, as far as I can tell do vary, dependant on time of day etc.
BRU - Ususally 10-12 on an ERJ-135
GLA/EDI - Largest load I have seen is 20 on the outbound, although I have seen only 4 on the inbound.
LHR is difficult to judge, as it is usually an A319, on stand 7 (Airbridge) but usually around 50. When an ERJ operates the service it is usually full.
I hope this helps in some small way.