At this point, you probably won't be able to snag one before Christmas unfortunately, from what I can tell UK retailers are all basically 100% sold out until after Christmas.
But your best bet would be to follow smileyhead's advice, call in to your local retailers and see if any of them happen to have some coming in stock soon.
Something you can also try is importing one from the US, stores here are getting stock much more frequently than elsewhere in the world, though this will likely bump prices over MSRP for you but it definitely won't be as much as shit scalpers. One good option would be to invest in one of those parcel forwarding services for US->UK, and then camp out on this page:
https://www.nowinstock.net/videogaming/consoles/sonyps5/ < and refresh the comments every now and again. There are tons of people who camp out basically every site and will make a comment when something goes live before any of the trackers send their notifications, so sometimes you can manage to slip in before bots take the site down and get one.
Another option related to the above is camping out on Sony's Playstation Direct page, you'll need a US PSN account and shipping address (hence parcel forwarding service), then just login to the site with your US PSN account, setup an auto-refresh extension for every 2 minutes, and then hope and pray you get into their queue system the second they go live with it so you're early enough in to get one in stock. It's how I managed to snag my own PS5, and it's basically the only site I know of that actually implemented some kind of anti-bot protection that works (after fixing a couple glitches, anyways)