Something to consider when listening:
Many London/Swanwick air traffic control frequencies are deliberately offset from the exact quoted frequency by plus or minus 5Khz. For example, I like to listen to London Welin sector frequency 130.925MHz however I cannot hear the controller side of the conversation that is UNTIL I tune up 5KHz to 130.130MHZ. I now receive the controller loud and clear.
If I wish to listen to Brecon sector, frequency 133.6 I can't actually hear the controller until I tune down 5Khz to 133.595.
You should experiment with some of your favorite London/Swanwick frequencies by tuning either up or down by 5KHz (assuming your scanner allows for 5Khz steps). You may be surprised and find a previously unheard controller coming through load and clear!
I have to admit I'm not sure of the reasons for this, maybe one of you can enlighten us all? In any case, the important thing for listeners are that you know about it and can experiment with some of those frequencies where you cannot here the controller side of the conversation. Happy listening!