Standard UK railway conditions of carriage for advance or similar discounted tickets as well as a number of other discounted fare types are that they are valid exclusively between the city pair for which they are issued and not intermediate stations. This has been so for as long as I can remember and is hardly new. This is a trade-off I can live with for paying £11 from London to Manchester or £17.50 London to Glasgow, journeys that would cost three times as much or more with a car even if the car rental was free and be much longer and more tiring.

Moreover, they bought their tickets on Megatrain, which discounts prices very heavily (tickets on their journey could cost as little as £1) against strict conditions (eg: no carriage of bikes, etc...).