Not a good plan....
The B100R, unlike most of our larger units, does NOT have a "universal" transformer in the US units. (Many Ampeg units DO have a 100/120/220/230/240V 50/60Hz transformer in all versions)
While an external "step-down" transformer will provide US voltage, it will not change the 50Hz UK mains frequency. US mains are 60 Hz, which allows a somewhat lighter transformer for the same output. The 50Hz requires a heavier iron core and/or different windings.
The 60Hz transformer will be "unhappy" at 50 Hz. Depending on voltage, it may draw excess current and overheat, even blow the fuse (or open the circuit breaker). You may "get away with it" much of the time, but if there happens to be a voltage surge, you may have a problem.
Incidentally, going the other way is OK. Virtually any 50Hz transformer works fine at 60Hz.