The F. magellenica (sp?) can grow to about 4' or more. I've seen photos of
some as a hedge in Sweden.
They're only moderately hardy (for my conditions) - they'll unlikely stand
a couple of weeks of -19C in a pot (don't ask).

You can grow them from seed actually, they're quite easy. If you do it this
summer/autumn/winter in a greenhouse or indoors you can have some for next
spring. South London shouldn't be too much of a problem with being too
cold.

that's one thing that worries me, often non hardy fuchsias last
for years here, but in the ground, not a pot.

You never know until you try. The young plants that survived last winter
were in pots in the garage and it went down to about -6C for a number of
weeks on end. The posts weren't insulated or anything. They died back, of
course but vigorous new growth appeared in the spring. They're about 2'6"
tall now.
The ones that died were outside, where it went down to -19C.

--
Tim C.