No control over days off, as they can force assign you work up to a week before the beginning of the month.
I don't think that is entirely fair. We've been living under so called "force assign" for nearly 12 months now and I've been stung only once, and I haven't been working to CAP + 3 or 4 every month or anything ridiculous like that. Most of the time I get the days off I need (I am about 40% from the top of the Airbus at LHR). Sensible bidding offers a level of protection. Nevertheless I do agree that there is no way to cast iron guarantee a day off without having leave booked. But then, draft in some form has always existed at BA.
I would concede that increasingly I never seem to get more than 2 days off in a row, July being a good example. There can be no question we are working as hard as I ever have in a flying job. We are however, hopelessly short of pilots and the situation will doubtless improve as a) more pilots are recruited and b) CAP necessarily starts to reduce towards the year end to avoid an overrun.
As a very junior FO you would, without doubt, find yourself working nearly every weekend and probably not having more than 10/11 days off a month during the summer months. That's how a seniority system works, at the bottom of the pile you get little choice over what you do. Quite how EASA will affect rostering at BA is a big unknown at the moment but it probably won't be for the better.
I support the various other comments made about BA being a good, secure job with good training and almost always you fly with good people. The camaraderie with the crew is non existent a lot of the time though and would probably be a big shock coming from a charter airline. That's not to say you don't have a good night out every now and then, but it's rare.
Average number of 4 sector days in a month is around 2 at the moment.
Hope that helps.