Not sure that I can really offer anything to this (but I just wanted to post as I am also new), I live in Holland (The Hague) and I recently bought an 1100GS. This has no mph on the clocks the same as the German and continental versions (Having two sets of units on the speedo is something unique to the UK I think), however I don't see why you would need to change the whole speedo assembly, why don't you just get some of those stickers that they used to use for so many grey imports (like my brother's old CBR400), if you stick them on, I see no reason why it should fail an MOT, and if it does, then you're only back where you were anyway, and speedo units are usually really expensive.
Also I think you might have to pay some import tax when you import it to the UK (you need to check this), but if, for example, I wanted to import a UK to Holland, I would have to prove that I had owned it for 6 months prior to bringing it over in order to avoid this.
Just my two pence, worth, and hi everyone!