For example 1V67 is an express passenger train heading for the Western region with 67 identifying that train.
More specifically the letter indicates the route. On class 1 routes, some letters are reserved for trains going between two or more regions as follows: S = Scotland, M = LNW or Midland, E = LNE, L = Anglia, O = Kent/Sussex/Wessex and V = Western. Although sometimes class 1 trains bear these letters when they're not interregional. Other letters just refer to a particular route or destination within a region.
Class 2 trains (stoppers and other local services; class 2 means "low signalling priority") do not use region letters if they happen to cross between regions.
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There are 6 trains today with 1A04 as their headcode. 2 of which both do Glasgow to Kilmarnock at different times.
In those cases the unique identifiers are G60022 and G60023 (UIDs consist of a letter and five numbers, can't remember how they work).
There are only 4,600 possible passenger headcodes (two classes, 23 route letters* and 100 numbers), whereas there are far more than that number of trains each day.
*Q, X and Z reserved for special purposes.