I can only speculate, but the general idea is that the market is monopolistic (tube) - represented by a single firm which provides the service, so is able to charge whatever price they want. That's the theory.
In practice however, the price reflects the costs associated with the product. In this case it is safe to say that the industry vastly relies on human labor - drivers, assistants, cashiers etc. which is quite expensive in the U.K. Also there's a ton of costs related to building the infrastructure and maintaining it.