It could possibly run in the CSCC's Modern Classics section - regulations here:
https://docs.wixstatic.com/ugd/838e8...67f228cc73.pdf
or 750MC Roadsports
https://www.750mc.co.uk/ugc-1/1/1/0/...4082acf631.pdf
or BARC Tintops
https://www.barc.net/wp-content/uplo...nal-190301.pdf
or NSSSC
http://www.darlingtondmc.com/data-fi...ulations-1.pdf
or MSV Trackday championship
https://www.msvracing.com/media/3774...one-signed.pdf
amongst others...
Or you build a car that complies with several sets of regulations and gives you options without committing yourself to one championship.
These regulations have to be read alongside the Blue Book available here:
https://www.motorsportuk.org/assets/...sedversion.pdf