I have no recollection of this series which was shown on Sunday afternoons in the autumn of 1987 (but was made in 1985). It's all on YouTube.
Set in 2020, John Woodvine is Prior Mordrin, the leader of The Knights Of God, a fascist cult, who have taken over Britain. A resistance is led by Gareth Thomas. Peter Childs only appears from episodes 8 to 13 as Brother Tyrell. Also in the cast are Patrick Troughton, Julian Fellowes, Nigel Stock and Don Henderson.