After asking on mastodon, I finally decided to get an 11" MacBook Air from ebay for £89. It arrived and worked!
It's not super fast: quad core i5 at 1.6GHz, 4Gb RAM, 128Gb SSD and a battery that lasted 4ish hours. Unfortunately, a UK keyboard. I decided to install Debian bookworm from a USB stick. With no fiddling at all, I had a new system installed within an hour (actually less but I was on a work call for the whole time and didn't see when it finished). I did review what was on the Debian wiki but this was pretty out-of-date.
During installation I chose to just go with the Xfce desktop environment, as I thought GNOME/KDE would be too much for an older machine like this one. More below on memory issues.