Our rail network just doesn't have the capacity to support significantly greater passenger volumes. A large proportion of peak-time services are severely overcrowded. Inherent signalling limitations mean that we can't fit any more trains into the schedule on those services, and we can't make the trains any bigger without spending billions on infrastructure improvements. Building more lines in a country as densely populated as ours is astoundingly expensive, as we have seen with HS1/HS2; the IEA estimates that HS2 will cost £670m per mile.
Demand-based ticket prices accurately reflect the availability of seats. The cheapest advance fares from Carlisle to London are £24 each way; the most expensive open single is £239. It doesn't make sense for rail operators to offer cheap fares on trains that will be filled at a higher price.
I think that UK is not that densely populated. Like at all. If Japan and China are expanding their rails networks (in their densest parts) the UK could probably do to.
Demand-based ticket prices accurately reflect the availability of seats. The cheapest advance fares from Carlisle to London are £24 each way; the most expensive open single is £239. It doesn't make sense for rail operators to offer cheap fares on trains that will be filled at a higher price.