London Info has three frequencies each with three transmitter sites so coverage is pretty good. As David says there is a known problem around the TOLKA, WAL, Valley area where both 124.75 and 125.47 coverage is poor.
All three frequencies are "cross coupled" which means that a call from an a/c on one frequency is almost instantaneously transmitted on the other two. This means that in theory you should be able to hear all the traffic on on all three frequencies wherever you are. This feature stops a/c stepping on other transmitions (sometimes!) causing even more chaos than normal on a busy day!!!!
I am not sure if D&D have other sites, I am sure someone else will tell you before I get back to work and enquire.