Interesting (seen from the perspective of someone who used to be a VAT inspector - me)
Arden Grange has a Breeder's Club. People who join sign a declaration to say the food they are purchasing is for working dogs and are then sold AG adult in a different bag and VAT free. I think it's exactly the same product as normal AG adult but has been bagged and labelled differently. As AG are "holding it out for sale" as a working dog food they don't have to charge VAT. It's not illegal. Other manufacturers do it, even raw food producers like Natural Instinct, whose working varieties are the same as the normal varieties apart from the addition of some extra Vitamin C.
Berrieswood are selling exactly that food even though the AG Breeder's Club rules state that the working food isn't to be sold on. However, if there's any dispute over that it's between AG and Berrieswood, not between AG and Berrieswood customers.
So if the Berrieswood price is good, which I'm sure it will be as it's VAT free, go for it