This is one I've used a lot - it's in Europe, but don't let that put you off. They've been excellent in my experience, and cover the import fees so there's no extra to pay. Just as easy as buying from the UK and, last time I had to return something, the returns were free. Look at the Gonso and Löffler gear - I've tried other brands who do larger sizes, but they're nowehere near as large as they claim!Exactly what I was thinking. So what now? 3XL is the max from the usual suspects. Where are you thinking of next?
AlpineTrek
This one is more tricky, as it's in the US, and you may end up with a large bill for import fees. Could be handy if you know someone who travels from there to the UK? Excellent range of kit in large sizes.
AeroTech