[IKE]

Along with what everyone else has said, "finishing the build" early enough to do hardware, software, and driver development is important. A lot of teams will build a second robot to accomplish these goals. Others will build a more simple robot and just get it built in 4 weeks, and then work on improvements.
When/if you finish the build early, use the additional time constructively. Work on scoring drills and skills. Improve software where necessary. Many teams have a comprehensive strategy on what the software will do vs. what the driver/operators will do. The more tasks the software does, the easy it is for the operators, but the more development time needed to get it right. For the 2011 game, this might include pre-programmed heights, score and release algorithms, acquire algorithms.... The same sensors to do these motions also are used for autonomous and thus increasing the reward. All of this requires quite a bit of effort and skills. There is a real art & science to tuning feedback control loops well and efficiently. practicing those skills in the off-season is a great idea.

From a driver perspective, imagine that you are playing a poorly tuned video game for the first time. Initially you will be very bad at it. After a few minutes, you should get the hang of doing some of the simple things. After an hour or so, you become proficient enough to get some timing to your actions, and begin executing strategy. After 10 hours, you are really quite good, and after 100 hours most would view you as excellent.
The problem is at a competition, you might have 2 practice matches with aroun 10 minutes of experience max. Then you may get 8-10 qualifying matches for another 16-20 minutes. That is only a total of 30 minutes maximum. This is why frequently you will see the 2nd round pick for the winning alliance get much better in the eliminations. Often this pick has only 30 minutes of concentrated stick time, but get an additional 15-30 minutes of stick time during eliminations.
For a team in the UK, I would try to keep the machine simple enough to finish early. A week of concentrated practice and tuning before ship day is equal to a tournament or 2 worth of experience with your machine.