The answer is that there is unspecified maintenance at my exchange which will continue for an undertermined amount of time, after which I should route via London.
I would wager with a response like that you are on the Zen backhaul. I have never seen any kind of BT maintenance which would require a single exchange to require terminating at a different PoP.
What exchange are you on? Remember FTTC has a concept of parent and child exchanged so while your exchange may not be directly on the Zen network, the parent exchange may be.