Conveying non-folding bikes (or indeed any other outsize luggage including non-folded push chairs) at peak times probably isn't a good use of carriage space. However outwith peak times, conveying bikes will generate incremental revenue from cyclists who wouldn't have traveled by rail if they couldn't take their bike. On long distance operators where journeys span peak and off peak then requiring reservations for the available bike places seems reasonable, though if there are spaces these should be available until 5 mins before departure.
In terms of bike accommodation; either a space with plenty of hand holds which can be used for standing passengers during peak times, or a space with flip up seats. Also, although 1 bike probably needs the same space as 3 flip up seats, 2-3 bikes can fit into much the same space.