After a call with the recruiter, I was sent a technical test (which I was told would be followed by a technical interview if things went well). This is the same test they use across the entire group (as far as I could tell) and they haven't changed it in at least a couple of years.
It asks candidates to write a Django application that implements a parser for a very industry-specific standard, which specification is, and this is written in the problem statement, fairly confusing to navigate and understand, and somewhat incomplete. You'll have a 3-4h time limit (they say not finishing by then will not be held against you - that's a lie, you'll be failed automatically), and at least a third of that will be just figuring out how to read the specification. In truth, a realistic expectation for finishing most of the test (i.e. only be missing some optimisations and a few tests) would be around 5-6h, so just ignore the time limit (everyone I know who've gone through this test does so).
Then, even if your submission is more than good, once you've submitted it it's a coin toss. There's no indication what a passing grade even remotely looks like, and I've heard people being failed for a variety of reasons which most would consider nitpicks (rather than whole reasons for failing an application), such as too many comments, or missing something not mentioned in the problem statement's requirements. Feedback which amounts to "we were impressed at the submission but we found a small bug and you're missing a small bit of doc we didn't mention in the requirements" would be something I'd expect to mean the submission wasn't perfect but we can talk about it during the follow-up technical interview, but apparently that meant I was out. As a reminder, I wasn't even applying for a senior position.
Honestly considering the feedback I'm almost glad I got rejected, considering how high the bar was for a non-senior position with the salary range I was told about during the initial interview (£50-70k). If the expectations are higher for non-senior devs than I've seen or experienced on senior devs in other places, then it feels like I've dodged a massive silver bullet.