I've never seen a testimonial signed by the Commissioner, himself... https://en.wikipedia.org/wiki/William_Horwood_(police_commissioner)
You usually got a certificate http://www.open.ac.uk/Arts/history-from-police-archives/PolCit/docVolCert.html (though usually signed by an Assistant Commissioner or below)
The earliest example I've seen was 1902 I think, but possibly earlier, and the MPS still issue a certificate on leaving (fairly similar), that is signed by the Commissioner, personally.
I've seen a few interesting Certs of Service over the past few years: I've held PC Sidney Miles' KPFSM (killed on duty), seen Supt Donald Swanson (of Jack the Ripper fame); and PC John Reginald Halliday Christie's (the crook) (when a war duties constable in WW2)...
All best
Zeb
Christie's Certificate of Service below