This is a great question.
The obvious answer is to then go to London.
But if you are unable to do so AND chave no mobile network, you can download the maps via avian carrier.
I do the same when I go on vacation. Take an old phone, no cell plan, just use the wireless at the hotel and take the phone as a map and camera. No cell plan means work can’t call me, map still works bc of GPS and bc the data is manually downloaded (under profile menu.)