The question asks for the lowest available flight level. Some basics - the transition altitude outside CAS is 3,000ft. The first fl30 on that track is FL30 and it is also the transiton level. The transition level is not available for use as it does not garauntee any separation from aircraft flying at the transition altitude. Therefore in this case FL50 is the first available FL on that track that provides any separation from the transition altitude.
Sorry but I do not think this is correct - where does it say that the transition level has to give any separation from the transition altitude? The transition level is the first available Flight Level above the Transition Altitude although from previous threads it seems that somes States do specify this but not the UK - I stand to be corrected!
Take a look at this thread:-
http://www.pprune.org/forums/showthr...ansition+level