Used cars in the UK are incredibly cheap as is road tax.
I was looking at an '04 182 in the Netherlands for a friend- asking price was nearly 6000 euros and road tax for the year a snip at 680 euros a year. The car was an utter shed that had been imported from the UAE and had terrible sun damage on every panel and the dash. So the dealer offered to cut as a special deal at only 5000 euros.
Simple supply and demand. Nobody in the UK wants old cars, most people just go from lease to lease these days and the ones that don't won't pay much for their cars.
Also it's a limited market, there aren't that many places that use RHD vehicles and nobody from LHD countries want them.