BR class 37, introduced in 1960. These have a distinctive body shell with noses at each end and this is unique amongst the locomotives still in regular use (although you might catch a preserved class 40 or 55 ('Deltic') on railtour duty). The rebuilt 37s which now run as Network Rail class 97/3 also share the same body shell, but these are painted bright yellow!
https://en.wikipedia.org/wiki/British_Rail_Class_37
Current main line operators of the class 37 include Colas Rail, Rail Operations Group (ROG) & DRS, plus HNRC have members of the class for hire to the main line operators. West Coast Railway Company (WCRC) & LSL both have operational main line registered class 37s, used mainly for railtours.
More than 30 class 37s have been preserved.
Tony