It might be an age thing, but as good as internet information is, a good "proper" book is my preference.

My current favourite is my RAIL ATLAS (Great Britain and Ireland) by S.K.Baker.

I've just updated to the 15th edition, and I sit in my chair with it when following YouTube cab ride videos. Some might see that as sad, but I find it fascinating from a railway perspective, and also countryside, industry and places of interest.

In addition to Philip's recommendations, if you look on the internet for British Rail Fleet Survey there are several volumes available, from 1 to 12 covering all motive power.
I think volume 3 will be a good starting point for you. It covers Production Diesel-Electrics Types 4 and 5. ( types relates to the power rating class of the locomotives

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.

Brilliant bit of research Philip, I'll be giving the sites a visit during these lock down television sessions. Thanks.

With you on that one Philip.

People often say something like "I went from Tilbury to North Pole jn" and I have to go away and look it up. I can't remember/work out how to do it, and then I kick myself when I see it.

It's funny, when I was younger I loathed History & Geography, and now I can't get enough. Just life's long journey I guess. ;)

Try this one as well,

British Electric & Electro-diesel locomotives 1992-2000

https://www.youtube.com/watch?v=DOY9y1ySH64

With the Don Coffey videos being based around Manchester - Leeds - York I've got to know that area quite well in the last few months.His captioned videos are the best and most informative.

A lot of junctions in that area are suffixed with the old (pre 1921 grouping) railway company names like LNW and L & Y. (London & North Western and Lancashire & Yorkshire)

I often leave a comment for Don on YouTube after one of his cab rides. Sometimes I follow the ride with goggle maps to follow train progress.

With a bit of time on my hands today, I've done a bit of research...
I am going to try and list the locomotive types in use on the present day railway, although some of the photos may be of preserved locos for illustration. Also, don't pay too much attention to the liveries, as some of these are not currently used and the photos don't show every different livery.

There are two main types of locomotives in use on Britain's railway network - diesel shunters and main line diesel & electric locomotives. In the past, every major station would have had one or more shunter on station pilot duties, but these days they are confined to marshalling yards and depots and are much reduced in number.
The BR class 08 diesel shunter was introduced in 1952 as a development of the LMS design (BR class 11). The class 08, 09 & 10 were outwardly similar in appearance, but the 08 was the most numerous totalling 996 locomotives.
https://en.wikipedia.org/wiki/British_Rail_Class_08
Current operators on the national network include:
Network Rail (NR), Great Western Railway (GWR), Scotrail (ScR), Wabtec, East Midlands Railway (EMR), Freightliner, Eurostar, West Midlands Trains (WMT) & Derby Works. Others are owned by leasing companies including Harry Needle Railroad Company (HNRC), Rail Support Services (RSS) & Railway Management Services (RMS) - these can be found on hire to mainline railway companies, industrial users or preserved railways.
A large number have also been preserved.

Tony

BR class 20, introduced in 1957. These locomotives have a distinctive long hood with a cab only at one end. Latterly the class 20s were used for Railhead Treatment Trains and for nuclear flask traffic, but they will be relatively rarely seen these days.
https://en.wikipedia.org/wiki/British_Rail_Class_20
Direct Rail Services (DRS) still have two class 20s in use, whilst HNRC have a fleet of 9 operational locomotives, plus numerous others either stored or withdrawn.
Locomotive Services Ltd. (LSL) also have 2 class 20s, which are main line registered and could be used on railtours or on hire to other companies.
Around 20 locomotives have also been preserved.

Tony