From what I know, Wattgate only make USA and Schuko type plugs. The UK plug Russ Andrews sell is a modified Clipsal plug. The only company I can think of that may be able to help is The Missing Link,
www.the-missing-link.net. Are you reterminating an existing cable?, or are you making up a diy cable?. The other problem you have is that most UK plugs are not designed to take 18-20mm thick cables, and the cable will be a tight fit as the terminals aren't designed to take heavy gauge wire. If you are reteminating an existing cable it may be worth just using a good quality adaptor which can be bought for aound £25.00 for a fully moulded adaptor, if you do use an adaptor you will need to make sure the plug is wired the same way a a UK plug, as USA and Schuko plugs are wired slightly ie earth, live, neutral and not earth, neutral and live.