I note that the recent bike listings such as https://www.sjscycles.co.uk/bikes/thorn-258261/ includes "Please note that we no longer have an open showroom." However, there's nothing on the website to indicate that the physical shop isn't open so maybe they need the showroom space for other uses.
There is no shop, other than online, I don't think they're geared up to sell you anything over the counter.
I recounted a visit there a few years ago, the thread will be here somewhere, I'm not going to read it, I
might have said I wasn't buying another bike. When you walk into the premises, it's wall to wall Bromptons and an office! You really wouldn't know it was the home of Thorn bikes, they were good enough to give ne a guided walk around, including upstairs to what might have been described as a showroom though it looked mostly workshop, I don't know if that's what's no longer open, it wasn't somewhere you could have a mooch around anyway.
I have no knowledge of their business other than as a satisfied customer, they have built a good reputation for parts, at one time getting good service and paying a little more for it, now they're frequently the most competitive on price as well. I have no way of knowing how big a part of the business is Thorn bikes, I wouldn't be surprised if it wasn't the biggest part.