Hi,
I live in germany and had the same problem with two old (1968...72) sunn amps. I did ring tube amp doc in worms and they told me it would be no problem with the old amps - cos they use big transformers (lot of iron). But they told me not to do this with new amps, cos they have smaller trafos! The 50Hz will cause a lot more heat, you'll will notice that your trafo is getting quite hot.
I had an 1200s (sunn/fender) with a service manual, so you could see that there were other parts in the power unit compareing to the us version. Maybe u get a trafo for a Fender Bassman 300 u.k. version as a spare part.
Greetings
Oliver