Very valid point about backup for emergency situations like in Spain and Portugal recently. It's my understanding that the UK does have an emergency broadcast network - not on AM but FM. It is made up of a big network of selected BBC local county wide stations transmitters which have backup generators on site. These are the transmitters that cover the biggest areas - not local relays. I don't know about how they backup the programme feed to the sites but it must use some sort of priority network or microwave links.