The national JICTAR ratings (up to) 1977 were given for "millions of homes" watching - NOT people (multiply by a factor of 2.2 to get roughly the equivalent of todays figures).
The regional ratings were given in percentages of the viewing public watching a particular programme for the population of that region . e.g. if the London JICTAR rating for a episode of "Doomwatch" was say 20 (the TVR) .. then 20% of homes were tuned to that episode with 80% watching ITV, BBC2 or not watching TV at all. The Stage/Television Today magazine had weekly JICTAR listings of the top 20 programmes both Network (national as millions of homes) and for all the 14 ITV regions (as TVR%).
With thanks to Martin