I doubt anyone could start by running too many services per day, but imagine there was a fairly regular service and the benefits for first class passengers with flexible tickets to just turn up and go, unlike at an airport which requires getting to the airport, clearing enhanced security (not really the same at St Pancras) and getting to the gate etc. The time saved there could more than make up for the extra time on the train, as well as the benefits of getting on the train and getting comfortable without the take off/landing preparation. If people considered the door to door times, I am sure they'd accept train journeys of 5 or 6 hours without much fuss. Of course, people rarely count the time taken to get to an airport, or even the cost of doing so. Obviously not everyone lives right next to St Pancras either, but I do think a city centre has many benefits (especially for those who may go to London for work, then travel from an office to St Pancras and onwards, and vice versa) and shouldn't be underestimated. As I said, to truly compete you probably need a regular service and that gives you the chicken and egg scenario. You can't just start with that, and if you had just one or two trains a day then the flexibility goes and people may just stick with flying.