Yes you are allowed showroom stock as long as the vehicle is unregistered.
When my MB car was written off a few years back, I visited my local Toyota dealer to check out the C-HR and there was a shiny metallic blue one in the showroom. After sitting in it/checking it out, the dealer came over and asked if I wanted a test drive in one sitting outside in the yard. Took a test drive, got back, and the dealer asked did I want the one in the showroom as it’s exactly the model etc that I would have ordered.
I was driving it away from the showroom 2 weeks later.