Recently the company Framework announced their plans to supply people in the USA and Canada with 2TB SSDs compatible with the Steam Deck but for people in the UK, Scan may be a good option.
Scan have listed a Sabrent Rocket 2230 1TB NVMe, which they plan to release on February 27th for £159.49 and you can pre-order it now. On their listing, they even mention Steam Deck as one of the compatible uses for the SSD, so it's good to see more stores begin to offer this tiny storage option.
Shopping around a bit, this does seem like a reasonable price as I've seen it going for quite a lot more across other stores. You could look to the likes of Ebay, AliExpress and more you never quite know what you're going to get (if they even turn up at all) so it's nice to see more legit sources with somewhat reasonable prices begin to show and over time we should see more of it, and see prices drop, just like we do with other types of SSDs.