Ben, BA does contribute to the pension scheme, I believe it's 5% if you put in the full amount every month....bear in mind that contributions are based on your basic pay, not your total pay (basic + allowances). If I were you, definitely opt in (the company are offering to put some money in your pot after all...), but for goodness' sake make sure you're saving money for retirement elsewhere....if you rely on the BA pension alone you'll be begging on the streets when you're 55....
Regarding parking.....BA crew actually use a car park 2 roundabouts away from Compass...the car park you're thinking of is for operational (ground) staff based at Compass & visitors. However, on weekends (when office-based staff vanish from the face of the earth...), you'll be able to park at Compass centre.
Availables do differ from standbys....you can get single days, doubles, triples or quadruples! Basically, you 95% likely to be working on such days....scheduling just haven't got round to actually assigning you any work at the time the roster was published...with anything more than 1 available, you can call scheduling to arrange a trip (if there are any in the system). Occasionally, if you have a single AVL at the end of a block of work you may actually get NON-OP which means you've been given the day off.
By the way, sorry about the long posting.....enjoy BA.