I think it's just sloppy advertising.
When you are in the UAD store the prices are clearly listed in your local currency. On the flipside, when I get an email offering me a $50 voucher, when I go to my account the voucher is actually £50.
I don't think VAT is solely responsible for the price difference. The UK market is generally more expensive and less customer competitive than the USA. It's been a very long tradition in the UK that dollar prices translate to pound prices ($100 price equals £100 price), which is slightly unfair and annoying, but it has been that way for a long time and covers a myriad of different products and different companies. I remember when music CDs were significantly cheaper in the USA than UK.