Many Chinese sellers use UK forwarding companies to hold their items and item location on listings is exactly that, where the item is and not where the seller is registered. This is all above board, and allowed. For seller's country of registration you can see that, from clicking and finding the feedback profile page.
However, if the item location states UK with say a 3 - 5 day delivery, and it then takes 3 weeks to arrive with a Chinese stamp, that is misrepresentation and can be reported, 3 vertical dots on all listings, right hand side brings to you to ... report a seller.
If sellers cancel due to out of stock for a reason, this brings about defects on their accounts, which can result in selling sanctions or selling limits.
@karelscars