It comes down to whether there prepared to export it as it may well be an ITAR item. I was after getting a mate ( who had a custom airsoft buisness at the time ) to import me a rail and after a few months trying we gave up. Does Brownells UK not have anything suitable? Id it is it is a very serious crime to export it without the proper paperwork which takes months and costs a fortune.
Its perfectly legal in the UK ( only pressure baring parts are licensed ), it may raise interest from Plod but just tell them what it for and they'll go away happy.