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have two class 20s in use, whilst HNRC have a fleet of 9 operational locomotives, plus numerous others either stored or withdrawn.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