Your seniority number is determined by the day you joined. If there is 25 of you on your course, then your seniority within that 25 is MEANT to be determined by performance, however, I ended up being junior to 2 people on my course with performance issues, so I am not so convinced by that claim.
Once you actually start flying this seniority is pretty much diluted by the 1000s of crew that have been there longer than you. It will only really affect you if you fly from someone on your course and you need to choose working positions.
As far as promotion goes, realistically, you are looking a good 5-7 years ahead at LHR (maybe less at LGW) before you are even allowed to apply. Your training will be then be a distant memory and your performance in the training doesn't really come into it. It also goes the other way, if you didn't perform so well in training doesn't mean you have less of a chance for promotion.
My advice would be just to get stuck in, pay attention, be on time, adhere to uniform standards and revise at night - just like any other airline.
Oh, and MY training was Mon-Fri but I wouldn't rule out weekends unless told otherwise. Cranebank is open 7 days a week.