A fun side-effect of their JANET (roughly analogous to nsfnet in the US) hostname pre-dating DNS.

I believe anyone can get .uk domains now, they don't have to be .co.uk - but this one's been anomalous since 1985.

bl.uk is one of the few original direct subdomains of .uk that existed before Nominet opened up to direct registrations in 2014. As far as I'm aware, bl.uk, parliament.uk, nls.uk (national library of Scotland) and nhs.uk were the only .uk domains that weren't under .ac.uk, .co.uk, .org.uk, .mod.uk, or .gov.uk before that point.