First join a motor club that takes part in / arranges sprint events. B19 or Sevenoaks Motor Club amongst others. Some useful info here -
http://www.sevenoaksmotorclub.com/sprints
Will need a non-race National B licence for MSA events, but this is just a case of paperwork, no training/test. MSA Blue Book has all the details regarding class structure, what is and isn't permitted in certain classes -
http://www.msauk.org/uploadedfiles/m...A_Yearbook.pdf