There are several!
I will be racing at the Chippenham Winter Indoor Championship (
CWIC). Big track, big turnout, competitive field.
I believe Maritime Racing in Chatham are running a Carpet Wars series.
The British Indoor Winter Series (BIWS) in Sheffield is making a comeback this year.
Stafford are holding a Mini Nats series.
EWS and Rug Racers in Essex.
etc. etc.
Plenty of good on-road/carpet series this winter to choose from.