That seems decent value, especially if you're going to be having a custom frame built too, the Thorn one is £50 more! Unless the stars align your builder may well end up making compromises in the frame geometry due to building around the pre-existing fork geo.
Plan is to acquire the fork, then have a custom rando frame built around it...
Thank you all for the suggestions.
Winston can build me one with nice steel, but it's £250.