God, I hate this. I used to do this all the time under UNIX, but I am so lost with CUPS.
This link might help. Warning: It's dense.
You need to replace the '#' with the octal codes for the pound sign.
It's weird. The number 3 shifted on the US keyboard is '#' and on the UK keyboard it's the pound sign.
It's almost as if you've loaded a different keyboard driver and the printer is interpreting the key as US instead of UK. Hmm.
Is there a UK version of the printer driver available? Just grasping at straws here really.