Someone posted on another thread that Al's in Blackpool was a good shop and they say they are a Jackson authorised dealer so might be worth a try. In my experience I have found advice and technical input from people in skate shops very variable, and I could not accuse many of them of being worth of the title "skate tech", nor have they been willing in my experience to take responsibility for a bad fit. To be fair, I think they are pushed into a corner by manufacturers and wholesalers/importers who don't want stock lying around they may not be able to sell.
I think the distance selling law is that you can send stuff back that is bought online unless it's a bespoke/custom/special order item. I would avoid buying online if at all possible, and try on first, even if it means you have to travel, but if you do buy online it may be worth establishing upfront whether the shop would regard it as a special order and what restocking fee they might charge in the event of a return.
Is there a special reason you want that make/model? Skates vary quite a lot in shape, as do feet. Until you've tried them on, it's hard to tell what's going to work for you and any advice about size you get from strangers on the internet who don't know your feet is going to be of limited value.