Does the system use the same network of transmitter towers as D&D 121.5?
Hopefully not, because London Info has dead spots over the Irish Sea where you might need 121.5!
There was a discussion of London Info a while back where it was revealed that one outlet is at Clee Hill, directly shadowed by Snowdon when flying near Holyhead. Possibly 121.5 is there as well, but perhaps someone can advise if 121.5 has other outlets, like on top of Snowdon for instance?
It's a strong argument for always talking to someone when in remote areas. You can't know for sure that 121.5 will work. In the Irish Sea Dublin or Shannon are always there, long after Valley etc. have gone home. (Maybe they'd respond to 121.5?). Otherwise, it's relay by airliner if you're lucky and have time.