For anyone interested the block that has been sold is 51.174.0.0/15. The remainder of 51.0.0.0/8 still belongs to the Department of Work and Pensions though none of it is publically routable.
Department of Work & Pensions. Ho Ho Ho ! No-one seriously believes the DWP uses a /8. The whole UK Gov can use the block for internal purposes as it was allocated at a time before RFC1918 was a thing and everyone worked on the basis you needed to get an IPv4 allocation for your IP network. Several other public bodies in the UK got smaller blocks too (you can find them without too much digging on Google) but most of these were never externally routable either. You can bet traffic arriving from 51/8 is not about any pension.
Background: there were some pretty forward thinking people in the UK Gov at the early stages of 'the internet'; Parliament got it's own TLD when it was a case of sending an email to be allocated a TLD and various bodies got IPv4 allocations before they even had any sort of working networking going on. There are bits of the stories around if you look hard enough, but I've never seen it pulled together - I'm sure it would make interested (and probably classified) reading.
I think it's a mistake to assume anything malicious or untoward took place. A /8 is quite a lot of addresses, but at one point it was assumed a) the total pool of addresses would last a very very long time and b) that every network connected device would have a publicly routable IP.
I wasn't suggesting anything malicious. [edit: OK, perhaps I should have suggested someone's name had to go on the allocation record and DWP would look most benign. My recollection is the allocation was once listed as being NHS but that may be due to one too many whiskies and/or conflating another allocation.]
I was suggesting it was a throwback to the very early days of the internet expansion when people had a much simpler understanding of how it would all work and were excited to get involved. I think there are some enlightening stories to be told from various UK governmental departments which are not normally the source of such forward thinking.
Until last year all uk addresses were in the format of domainname.x.uk, where x mimicked the US tlds, .org.uk, .gov.uk, .co.uk (instead of .com.uk), etc.
Background: there were some pretty forward thinking people in the UK Gov at the early stages of 'the internet'; Parliament got it's own TLD when it was a case of sending an email to be allocated a TLD and various bodies got IPv4 allocations before they even had any sort of working networking going on. There are bits of the stories around if you look hard enough, but I've never seen it pulled together - I'm sure it would make interested (and probably classified) reading.