Motorsport UK have published changes to tyre rules dated from the 1st January this year. Blue Book section L3 has become "Tyres for all Rallies".
See pages 5 & 6 here:
This finally formalises the "No List 1C" rule we have had on local targas (due to changes to EU regulations).
List 1C tyres are not permitted, and List 1B are not permitted in 2020 (so use them up this year).
Tyres marked for "competition use" must be in List 6 (current forest rally tyres), or in the FIA List of Asphalt Eligible Tyres: https://www.fia.com/file/73133/download/9274?token=qJ5SIep_ These are current tarmac stage rally tyres.