Okay, took the plunge and paid my tax...

It requires your address and will calculate the amount based on the country specified - probably verified against your cc details as well.

However... You are free to choose your currency.

Setting it to UK pounds it calculates as £143.96 + £28.79 VAT = £172.75 total
However, setting the currency to US dollars it calculates as $199 + $40 VAT = $239.99 which, at the current conversion rate, is £158.20

They are using an exchange rate of about 1.38 dollars to the pound when it is currently about 1.52 - I know they probably want to standardise their prices but when was the exchange that low?

I.e. either they are doing something wrong or are applying one heck of a mark up on the exchange rate. Guess which currency I chose to pay in.