I think that's a bit of a leap.An HM number usually denotes an HM Government asset. Most likely this has come off a set of HMG keys and the number is, indeed, the number to call if you find it.
I think that's a bit of a leap.
Firstly, the tag belongs to computerquoteinsurance, which appears to be an insurance broker in Hants. Not sure they'd be entrusted with government assets, really. Secondly, if I had a set of keys to a government asset, I'd be rather more discreet about advertising them as such. So I really doubt the 'HM' prefix is what you assume, here.