I have had a response from Avon Uk, referring me to vintagetyres.com, which may be of interest as their website specifically includes Bristols.
Vintage Tyres: For all Vintage & Classic Tires +44 (0)1590 612261
Thank you for your email.
These tyres are only available via Vintage Tyres based in Beaulieu, you can find their contact details here
Vintage Tyre Supplies | Avon Motorsport
Kind regards
Christopher Askew
Technical Sales Engineer