No, bookings cannot be cancelled online. However you may apply for a refund under conditions specifically allowed for in the National Rail Conditions of Travel.
If you wish to cancel your booking, and your application meets these conditions, please read the following:
Where claims should be directed:
Application prior to travel: You should forward the booking confirmation document that was emailed to you when the booking was made to firstname.lastname@example.org stating that you wish to cancel your booking. No email application will be considered unless the original booking confirmation is attached.
Refund administrative fee: A fee of £10.00 will be levied on all refunds whether application is written or electronic.
Time limits: Your claim for refund must be made within 28 days of the expiry of the ticket's validity, no application will be considered after this.