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I'd certainly not regard it as unreasonable - but unfortunately it isn't permitted.
In the days of British Rail, you could take 'any reasonable route'. However, this changed upon privatisation so that - on paper at least - you instead have what are called 'permitted routes', as set out by condition 13.1 of the
National Rail Conditions of Travel. This says that (subject to any route or operator restrictions shown on your ticket) you can use any of the following:
- Direct trains;
- The shortest route;
- Routes permitted under the Routeing Guide.
The Routeing Guide sets out a wider range of permitted routes, including routes within 3 miles of the shortest route as well as 'mapped routes'.
The mapped routes in essence allow travel via routes that don't fulfil any of the above criteria but are still regarded as acceptable. For Crewe to London, the applicable maps and map combinations (you can
look at them here) are BV+GC, LC and RJ+RG. These respectively allow routes into Marylebone via Birmingham and the Chiltern route, Euston via the West Coast Main Line, and Paddington via Reading.
St Pancras isn't permitted under any of the above rules and therefore travel via Derby isn't permitted - nor is travel into Waterloo or any of the other London stations.
Obviously these rules are somewhat complex so a simpler way of checking is to use a journey planner to see whether you are offered a through ticket via the route you want (you might need to specify 'via' points). If you are, and the route of that ticket is the same as the route of your season ticket (e.g. Any Permitted), then it's valid.