Boopop
Guru
In my experience, at least at Leighton Buzzard railway station you can usually ask for two bike reservations without proving you've bought any tickets. Last time I got some grief from the staff member but I said I'd done it before (this time I was just asking for one bike space), but he printed out the reservation anyway.With Avanti it can only go on pendolino services (so ok from London to Glasgow) but needs 2 bike reservations so 2 tickets.
It's ridiculous that Avanti expect you to buy tickets for two seats on their trains if the only thing about your journey that would imply two people travelling is a larger bike, especially given that they do not charge for bike spaces. I own a tandem but several of the people I'd ride it with live several hundred miles away. I'm not going to let Avanti charge me double just so I can move a bike. I wouldn't mind paying £5-10 extra but a double fare is extortion.