As far as holdsworths go I would first try to find the year it fits in, then find the model sold in that era that closest matches yours. Keep in mind that not every model had a name on the top tube when you start going back in time.

It has the same headtube lugs as my 56 zephyr but I haven't seen those lugs on a non-path model.
Do you have a picture of the seat tube cluster?

Edit: also the frame number on my 56 is 15809, which would put your bike right around 1960