This certainly looks like a genuine Holdsworth from the 1970s. For sure it will have been made using 531 tubes for frame and forks. Normally a Reynolds transfer would have survived at the top of the seat tube, plus often smaller transfers on either side of the forks just below the crown. But maybe not if it was carelessly resprayed at some time. The wrap-over seat stays are particularly nice and will add to its value, as will the real name badge (ie not a transfer). Look at the frame number, or ask the seller to tell you what it is (under the bottom bracket). Then go to the Classic Lightweights website and then to Holdsworth and eventually you will find out whether it was a shop-built or factory built Holdsworth, the former being much more valuable. Incidentally, it looks like a track frame, which means the width between the ends will not be wide enough to take a wheel with gears. A shop can widen them, but it is not an ideal thing to do with a frame like this. So you had better be sure you want a single speed bike before buying this one. Personally I would pull out if the bidding goes over GBP 150, you are only paying for a frame here, and it will cost quite a lot to restore.
