I remembered viewing a couple of diesel loco guides on YouTube, so I've gone and found them again for you.
1) British Diesel locomotive Guide 1970 - 2000 Classes 03 to 37
https://www.youtube.com/watch?v=twarSFjLI7I
2) British Diesel locomotive Guide 1970 - 2000 Classes 56 to 67
https://www.youtube.com/watch?v=Z1EnNQG7tKk
That will get you started for the older, and heritage classes. Even class 20 and 37 still get to be seen in daily use.