I too was thinking "Holdsworth" but it's not a slam-dunk (excuse the basketball expression). There were a number of other British builders that nearly copied a number of the most popular Holdsworth models, including the Mistral and Super Mistral. Ron Kitching (the "K" in MKM) did sell something like that, as did the "other" Butler: Geoffrey Butler and I'm sure there were others...
Speaking of Butlers, anybody know if a Claud Butler (actually another Holdsworth Brand after 1959) from this time-frame would have used a metal badge versus a decal on the headtube? I have a feeling this might explain a thing or 2 (like the serial number which seems sequential, not a Holdsworthy or Holdsworth-Putney-shop norm) and perhaps the curved brake bridge and different braze-ons as well.
Maybe.