repotting

I would say definatly spring,or you could lose the fuchsias to frost.

You might get away with planting the fuchsias now as you live in London but safer t owait until spring.

if its a hardy Fuchsia you could do it now and put the pot in a sheltered spot.

You could risk it, but even hardy fuchsias are sensitive to cold through the roots, so more likely to die in the winter if they're in a pot, whereas in the ground, they survive, unless they're tender, when they won't survive at all outside during winter, in a pot or in the ground.

Anyway, they lose their leaves, so they won't be very attractive through the cold months. Why not put an evergreen plant in the middle - a small Skimmia, a Euonymus, or a Photinia 'Little Red Robin' for example??