Virgin Rosters
The Virgin rostering system is based on an equitable division of work based on a 750-hour contract. A few pilots have limited-time variations which allow them to work more or less than the standard 750 hours, but they are in the minority.
The roster is allocated to cover the workload that exists. Other than training, all flights are allocated on a more or less random basis, but to fit within the hours limitations. Bidding for particular flights is possible, and bidding for days off is possible. Bidding against particular flights may be possible, but would be frowned upon - after all, why would you join if you don't want to do the longer flights? That said, a 'trip swap' system exists.
The system that capped hours at a certain level per roster month is being scrapped; other than the CAA (CAP371) and union/company agreed limits (minimum time between turnrounds etc.), the only hours limit applied will be your contracted hours. This is intended to give more flexibility, though the average month will still have around 70-80 credit hours.
The rosters given above are fairly typical, but can be altered significantly if training or leave is scheduled in the month. My last month (July) had 5 East Coast flights plus a two-day sim session. That's about as busy as a month can get for a line pilot. This month, I have three trips and 19 days off - a bit more comfortable!