I regularly take bikes on trains when i go away for the weekend. Primarily Virgin West Coast and East Coast Mainline. It used to be that you just rocked up, jumped on with bike and went, but since the middle of the year I've always been asked for reservations, East Coast won't let you on at all without a reservation.
The Virgin Pendolinos are a pain because you can't open the luggage compartment without a member of staff. If you don't have a reservation, they won't turn up to let you out, and as I found out yesterday, if you do have a reservation, sometimes they won't let you out. Fortunately the train terminated at Preston, or I'd have been fuming, but having my bike locked on did mean I missed my connection.
The swines!