hello friend.
yes you can get tax at ferry boothes in dover or go to transmarck when you get off boat for it. remember in Austria the new maut started jan 1st.
Stop at the first petrol station when you cross the border and get a “go- box” it costs 5 euro but you have to credit it with at least 45 euro when you get it. very eay and simple to use (unlike the german effort). there’s lots of info on here about it and the austrians published a good guide to it which you can pick up down there and at petrol stations near the austrian border. they accept most of the big fuel cards and cash aswell.
cheers kenny