Sharkey,
For a moment I misunderstood you, and thought the power transformer of your F-2B was outboard. But it is obviously not the case. Rather, you have to use an outboard step down transformer to run the preamp from 240 volts. It's clear now.
My new F-2B has a power transformer, which (like most) has twin primary windings. Whether the primaries are connected in series or parallel will determine if the unit runs on 100-120 or 220-240 volts. It is likely that the same arrangement exists in your F-2B. As far as I can see there is no internal or external voltage selector switch, this is hard wired inside the unit. A tech should be able to make the necessary adjustments in about 15 minutes.
A the same time, however, it may also be a good idea to install a new power chord that has a UK plug and an earth wire on it and connect that to the chassis. If I can see it correctly on your photos, at the moment the power lead is only two core (no earth). Today Alembic supply all their mains operated gear with an earthed chassis. It is just safer that way. Once again, this is not hard to do, but consult your tech.
Cheers,
Peter