Well, kudos to Dark Side Toys for stepping in like this - there has definitely been a surge today.
I wonder how this is being done/accomplished. I would think it's most likely that they have a contact/business-partner in the US (or Canada) and each time someone orders the Sail Barge from their website, they have that individual place an order on HasLab - since the limit is on 5 per order and not on the number of orders, their business partner in North America could just keep doing that over and over again. HasLab will then just ship the bulk of them all to a single location and then Dark Side will just mass ship all of them from North America to the UK and then from there ship them around Europe. The "mark-up" is probably them knowing exactly how much tax/tariff there will be at each step in the process, so while I'm sure they're getting something for their trouble, I doubt it's much.
So definitely a win-win for them - if I lived in the UK, I would strongly be considering using them as my exclusive online retailer for all my Star Wars purchases.