Mesa simply export their kit to Westside distribution in Glasgow AFAIK. It shouldn't cost them any more to export than any other US manufacturer. And yes I do know (albeit a little) about international distribution costs via my day job.
Let's put it like this: if Jim Bergantino can export his kit to the UK with broadly a one $ = one £ rate allowing for VAT, shipping and customs then why can't Mesa?
The excuse that Mesa have been giving is that they don't set retail prices in the UK. (See lengthy discussions on this earlier in the thread). I don't buy that excuse, as you'd expect at least one retailer to break rank if there were "super profits" being made at that point in the supply chain.
Which is a shame - a lot of us on this thread, me included, are big Mesa fans. My M6 is still my all time favourite amp.