Welcome to the forum!
Holdsworthy were Campagnolo's main (I think) UK commercial connection for a number of years. As a framebuilder and retailer under the Holdsworth name, they fitted a lot of Campagnolo to their bikes and sponsored heavily with frames and groupset parts in the 60s, 70s and into the early 80s.
There is an interesting history of the company here:
So - depending on the time at which the frame and group were made, likely the freewheel (not a cassette at the time) would have been something like Regina or Everest. Again, depending on the vintage, it might have been 5 or 6 speed and the traditional racing ratios were often "straight through", so 13-14-15-16-17 or maybe "stepped" 13-14-15-17-19
Finishes on sprockets were typically flat silver - not chromed.
Unfortunately the shop closed late last year ... but there are sources of information out there which can identify your frame, it's age and from that, you can probably, with a bit of digging, get an idea at least of what parts were likely to have been fitted so that your restoration will be period-correct, if not an absolute restoration of that machine as it left the shop.