No, I do not have the answer, but maybe something in the below will help you find out.
Terry Thomas, who indeed appeared on ATV's Angling Today in the 70's, does not appear to have notice of his death posted in the sources I have checked, including the Sunday Times obituaries for all UK counties. He appears to have published Casting: A Text Book of Fishing Casts in 1960. I have no idea if its flyleaf would contain enough information about the author to allow you to narrow the search by knowing his county of birth. Even recordings of the old show might not offer any biographical information.
Perhaps a trial membership at the following site might pull up some information but you would have to search using a wide range of years and might end up with merely a list of people you could not narrow down without further information. If it gave a location of deaths, you might then be able to track an obituary in newspapers covering the relevant counties. death?kw2=Find%20UK%20death%20records%20instantly!&Location=UK&gclid=COLgoLmE3sMCFUcHvAodcKcAtQ">http://www.archives.com/uk/search/death?kw2=Find%20UK%20death%20records%20instantly!&Location=UK&gclid=COLgoLmE3sMCFUcHvAodcKcAtQ
I have no idea if contacting Chris Tarrant's agent, Paul Vaughn, with an email enquiry would result in any response from Chris about Terry. Chris became a notable TV presenter.