These days £400 gets you some superb basses new, too many to list realistically, that will all be great.
There's an even better selection second hand.
It all depends what sort of thing you want, traditional, modern, solid, hollow, fretted, fretless, 4,5 or more string.
What's the best sub £400 bass? Yes.
But get the BB424, if you can squeeze a tiny bit more into the budget or be patient, you could even get a BB1024. 🙂👍