Those are the early type Nervex Pro lugs, I believe. More info here:
http://www.classiclightweights.co.uk/co ... ervex.html
The lugs may have been slightly modified or cut away, which was not unusual. What is unusual is the sloping fork crown - the lugs suggest that the frame is early to mid 50s whereas a sloping fork crown is more typical of a later period. Perhaps the forks are not original.
There is loads of info on Holdsworth cycles here:
and also on the classic lightweights website above.
1996 TJ Flying Gate, 1987 Rotrax, 1969 curly Hetchins Magnum Bonum, 1956 Bates BAR, 1955 Gillott, 1955 Claud Butler Avant Coureur Special, 1953 Ernie Clements Special Course, 1950 Granby, 1948 FH Grubb lugless, 1938 WJ Hood Palace Special