Just noticed that TfL have recently published a report about onwards travel from the main Central London rail termini, which you can find here. It looks at who uses the termini, where they are going (in terms of location and trip purpose) and how they get there. Here’s a chart of onwards distance travelled by mode:
One of the more interesting findings is the make-up of the group of people that usually cycle their onwards journey: “Cycling is dominated by a particular demographic. Eighty two per cent of cycle journeys are made by men and 60 per cent of cyclists are aged between 25 and 44”. The new Cycle Hire scheme doesn’t seem to have changed this much.