The 51.* addresses are in fact heavily used by DWP, but only
internally. The best bit is this: for security reasons, there is a policy
that in any communication, the leading octet of all such IP addresses
must be redacted. Not like it's a matter of public record or anything.
I did once toy with the idea of printing out the XKCD map of the IP4
address space, write "you are here" on it and pin it to the wall near
DWP data networks teams, but I didn't think it would go down well.
And someone else isn't interested in more rearranging of deckchairs either:
It doesn't matter, save your energy for converting to v6
There are few dregs like this around that though not visible on the
internet are used - many large ISP have hijacked the unadvertised spaces
for use behind their own NATs. You can't see them doing this but if you
use the space on the internet you'll have problems with their customers.
If these spaces were recycled you'd spend many months trying to get them
clean for use and then they'd be used up in a few weeks, we'd still be
out of space and we'd have yet more to go back and convert to v6. People
have been dragging their feet on v6 migration for eyars, they only have
themselves to blame when they are stuck by v4 running out
tldr: It's allocated, it's not available, nothing to see here, move along.