RCM was Radio Counter Measures. We now refer to it as Electronic Counter Measures although RAF RCM assets from WWII included what we'd now know as EA, ELINT, COMINT, even active comms spoofing.
When you look at what 100 Gp did in the war, it could be claimed that the RAF wrote the book on SEAD, Compass Call and SIGINT.
Regards,
MM