Many modern amps have there mains transformers wound such that they can be wired for 120V or 240V operation, in which case any competent tech would be able to perform a conversion (thus saving the inconvenience and extraweight).
Another alternative is to buy a replacement export transformer and have this fitted. I've done this for Fender amps and it still works out cheaper than buying one in the UK (even with the charge for fitting).
Obviously if you convert to 240V operation you should reduce the mains fuse to about 1/2 the value for 120V operation.
Export transformers are usually larger as UK mains is 50 Hz rather than 60 Hz in the US (you need a larger iron core to avoid saturation).
If you do get an external transformer, the VA rating should be above the power consumption of the amplifier by a reasonable margin, say at the very least 25% more, although the bigger the better.
