Most main line long distance trains, ie Virgin, Great Western and the East Coast Main line have luggage compartments at one end of the train with some bike racks fitted in. You need to book them on, but there is no charge. On South west trains the bikes go inside the carriages with the bike symbol next to the relevant doors. Others vary but you can normally get your bikes on most trains, with the exception of commuter routes which have restictions on the times you can travel with bikes during peak times. Check with the T.O.C. before travelling.