Tv Audience Figures

[DeeLicious]

is there a company and/or website which has audience figures for British tv shows from down the decades?

[Buenchico]

The only easily accessible source of such data is BARB.

Their website allows you to see the top 30 weekly programmes on each of the main channels from 1998 to 2018 and the top 10 from a far wider range of channels over the same period:
https://www.barb.co.uk/viewing-data/archive-tv-set-viewing-data/

You can also see the top 10 programmes in each year since 1981 here:
https://www.barb.co.uk/resources/tv-facts/tv-since-1981/

If you needed any other information (for, say, a book that you're planning on getting published), you'd need to contact BARB to see if they could help you.

Kantar Media has a database of programme viewing figures going back to 1992 but it's not in the public domain. (You'd need to ask if they could assist you in their research):
https://www.kantarmedia.com/uk

Similarly, RSMB holds data from 1993 onwards (but, again, it's not in the public domain):
https://www.rsmb.co.uk/