I did a side-by-side with the B1 and M2 a few months back when looking for a pair for the living room. The M2 had more detail, offering a clear and crisp sound. The B1 had amazing bass extension in a more laid back presentation. It also cost less at $175/pair.
During the audition, I also listened to the B2. It was clearly a step up, providing a fuller, richer sound. I've never heard the M3 so I couldn't comment there. In the same shop, I also heard another British brand in the same price range, the AE Aegis Evo One. Very smooth as well, but with less bass than the B2. You might like to consider it too.
Good luck.
