I don't think it is a question of Hymer giving a monopoly. Hymer are testing the UK market and as such will start small and grow if the sales are there. Most European suppliers see the UK market as important, but in terms of sales it is small to what they can sell on the mainland. Plus the desirability to make vans more attractive to the UK buyer, eg door on nearside , oven etc does influence production lines and costs. You can buy your Hymer in Europe and get full after sales service, but of course the electrics won't be the UK type (not a big issue) and the door will be on the offside.. You would probably also have to return to the European dealer for any warranty work.
Being an island nation has advantages, and disadvantages, and this is one of the latter. Keep buying Hymers and more dealers will appear.