If you want to avoid customs duty it's best to ask the distributors in the UK (Gendan, ilexa, Marshalls or V-Tech) what their upgrade terms are, see https://www.ross-tech.com/distributors.php
But as Dave mentioned, if you do any of this directly with us, you'll need to factor in the cost of shipping, as well as any duties & taxes that Her Majesty will want.
You might find the best value way to upgrade is to sell your existing interface on eBay or similar, buy a 10-VIN Hex V2 and upgrade to unlimited via rosstech rather than a UK supplier.