I just had a scan through the info link you put up, Bowell, but I can't see anything in it about changing positions and reducing pay. :?
Gusto, I rather imagine, but I have no idea if it is the legal position, that if they change your job, they should not be allowed to reduce your salary. Have they given any actual reason for their refusal to let you work off shore, or have they made a generalised assumption that it would be dangerous either to other people, your environment, or to you?
If you look at page 9 on the UK advocacy pack that Bowell has linked to, I think that might be the most relevant bit. The Health and Safety at Work chapter. The bit that matters is where it says that the risk assessment has to look at the individual and not make assumptions about how the condition might affect you. They have to look at the reality. I don't see that there can be any need for you to prove you have been controlled for six months: surely the onus is on your employer to prove that it isn't and that you are, therefore, a danger to yourself etc.
That's just my opinions on it though, and not legal knowledge at all. But maybe the thoughts are something you can gnash your teeth at them with.
Many professions that were barred to diabetics years ago are now available to us. Even down to the emergency services - although I have to admit I don't agree with that one, but that's another story. I am now training to be a driving instructor, for instance, but I was not permitted to do that until recently.