I would be amazed if CC were limited to 900 hours of work a year. I believe it is 900 hours flying supplemented (I assume) by a significant amount of other work.

I'll try this for one last time then I'm giving up. My perspective is that full time working relates to being available for work when your contract of employment says you should be available. As an example, for me full time work is Monday to Friday 8am to 5pm for 45 weeks of the year. If I don't turn up for those 225 days then I haven't worked full time.

In the scenario that you have posed, the answer is dependent on whether BA have said turn up in weeks 49 to 52 because we have some non flying work for you (in which case not turning up would constitute not working full time), or if BA said we have nothing for you, feel free to have 4 extra weeks holiday (in which case you have worked all the time that BA requires so are full time).