Managed to get one! Ordered on pre-order from Game, and it's either meant to be shipped on or arrive (this doesn't seem clear) next Friday.
By the time I'd got through the queue, the console-only and console+Horizon options had gone - ended up with console+horizon+extra controller. At £140 that's a bit more than I wanted to spend but not the end of the world.
I've still got Horizon and GT Sport to finish, plus Tomb Raider (3, or whatever) and Red Dead to play through on the PS4! Might actually sell the PS5 Horizon to offset a bit of the extra cost, but I think I'll find the new GT difficult to resist in a few weeks...