Morning,
I suspect that the value would be very much dependent upon how much work you want to put into selling it and how long your are willing to wait for a sale.
If you sell the frame by itself it is likely to sell fairly easily at up to £100, if you are willing to wait a little bit and possibly pay a few repeat listing fees, contact local bike clubs etc £100-£200 should be easily achievable and £250 if you wait for that one person who wants it. If it is in as good a condition as you say. You might be able to get a bit more if are prepared to demand more but for over £250 it could turn out to be a long haul.
New a similar frame is well over £600
http://www.bobjacksoncycles.co.uk/product-category/off-the-peg-frames/ but this is a very limited market.
If you build it as a complete bike then a lot of people will be forced to buy bits they don't want, such as the 5 speed drive train.
There is a real but limited market for the components where they have value rather than being unwanted, if someone wants say the rear mech then you can list it at say £40 and are being very reasonable or £100 and greedy if it is unused.
As a complete bike I would expect it to be a fairly easy sell at up to £250-£300, someone who really wants it maybe £500-£800 but finding that person is going to take time.
Its easy to say that this is a classic, but there are a limited number of people willing to pay for classics rather than just drool. :-)
By 1977 a frame with the Holdsworth name wasn't a one off craftsmen made, I had one bought from a small chain called The Toy and Cycle Centre.
In many ways this bike is similar to
https://www.cyclechat.net/threads/raleigh-royal-23-5-frame-absolutely-stunning.263916/ although the Raleigh has unspecified material for the stays.
I am curious about the 531 sticker and the pink 531 does anyone know if this is just fading?