If I had £700 burning a hole in my pocket, I suspect I would go and talk to Bob Jackson. I have one, got it resprayed a few years ago and kept being asked about my new bike on Audaxes. Its at least 20 years old, 531, and just is *right*.
Other frame builders are available – looking at the options is part of the fun isn’t it?
I’m not entirely clear what specific ‘purpose’ you are looking for, but would suggest that well built, nicely painted, classic lines etc will stand you in good stead.