I'm not sure which London Station you want to leave from, but it will cost £36.50 return at an off-peak time from London Victoria station. If you travel at peak times, it may cost you more and you might have to book some time in advance.
The National Rail website is a good place to check fares and timetables:
Have fun! over 8 years agoAnswered by Lou Hutton via WorldNomads.com