I want to travel to roscoff by train. However, the section from morlaix to roscoff is using a bus. In the UK you can't take your bike on these buses, or any bus. Is this also true in France?
It depends. You need to check the specific line, or announcements about the replacement service. Look up the information booklet (“dépliant” or “fiche horaire”) about the line you want to travel on. For a regional line, go to the TER website. The summer 2016 booklet states
Les vélos sont acceptés à bord de certains cars équipés de rack à vélo. Service gratuit, sur réservation. Renseignement auprès de Contact’ter au 0 800 880 562.
The winter 2016 booklet states
Les vélos ne sont pas acceptés dans les autocars TER.
So on this line, in normal service, you can't take your bike on a bus. During the summer some specially-equipped buses allow bikes, but you have to check first and you must book in advance even though it's free. You have to phone. This is a rather touristic spot but many tourists would be French so it isn't guaranteed that the person answering the phone would speak English.
I found a bit more information on the local transport website and on some tourist information sites. Booking opens 48 hours in advance and closes at 17:00 the day before departure. The phone service operates Monday to Friday 7:00 to 20:00. The 0800 number is free from French phones (“numéro vert”).