I'm not sure about rail passes, but for TfL, your office hours might mean you'd be paying off-peak fares. If you work 2-8, you'd miss morning peak (off peak starts at 9.30a) and evening peak (off peak starts at 7p). That might not make a difference for rail passes, but is worth looking at since it might make a monthly card more expensive than pay as you go.
For TfL, annual passes are the cost of 10 monthly passes, so you get two months free.
As someone else mentioned, fares usually go up once a year so try to buy a pass before the price hike if you can.