Could be an escaped one. Looks like it may have those leather things on its legs. Good pics and a great bird.
They do well in the UK as escapees. They are hunters more in the goshawk mould than buzzards or kites (like the kite photobomb in the 3rd pic BTW). BB Mag Rare Birds Breeding reports list the occasional breeding by escaped Harris Hawks. Saying all this, the falconer who lost the bird I'm sure would be happy to get it back. One more issue with espaced falconers birds is the jesses (technical name for the leather straps on the legs) can get wrapped around things and trap the birds.