Theheadphonecompany is UM's agent in the UK, and yes they do provide the service at 380 pounds. To be honest if I were you I would not be pulling the trigger, doesn't look like anyone has tried it out and thus virtually no opinions about the outcome. In case you do want to give it a shot I think it'd be best to send it directly to UM global and let them handle the whole process instead of going through THC, they charge way too much. I might be wrong though, but I don't think you'd get charged custom duty for it in case you do it yourself, since it's essentially an upgrade and not exactly like buying a new product.
This price seems a bit too much... :) More what I spent for IE80 with cable and tips... If I would give this amount of money for upgrade, than I would definitely sell my IE80 and buy sg in the 6-700 pound price range, like the new IE800. But its not the time for this right now. (Not to mention my budget.) :)
Thanks, by the way.