Under British Rail, it was whatever route was deemed 'reasonable'. It didn't really matter as long as you were going more or less in the correct direction, as the money was all going to British Rail.
Under privatisation, the money from fares needed to be divvied up between umpteen private operators, so the concept of 'permitted routes' was invented, with the money for each flow split between each company that a ticket would be valid on, by a percentage of approximately of how many passengers would use each company's trains with that ticket
Under privatisation, the money from fares needed to be divvied up between umpteen private operators, so the concept of 'permitted routes' was invented, with the money for each flow split between each company that a ticket would be valid on, by a percentage of approximately of how many passengers would use each company's trains with that ticket