Frodo,
I'm not entirely familiar with the use of SE cable because I don't use it up here in IL. But if you put a main breaker immediately after the meter, then technically your run down to the basement would be a feeder and not service entrance. Can you use SE cable for anything outher than Service Entrance? (sorry I'm not familiar with it.)
A couple of different ways to consider doing it, all would depend on different variables.
Could you change the 125 amp panel to the 200 amp and then feed off that to a 100 amp in the basement?
Could you put the 200 amp meter on a wall just outside the basement and put the 200 amp there, and then feed th garage with 100 amp?
Get a combination meter with two breakers (this would technically be called the service, so pay attention to how you ground and bond) and feed each with its own breaker? (2) 100 amp breakers..... or a 300 amp service with (1) 100 amp breaker and (1) 200 amp breaker.
I don't have the NEC reference, but you can't put a breaker in a panel that is larger than half the size of the main. (somebody please correct me if I'm wrong)
Good luck.
