Even if it wasn't legal, there is a case law (name escapes me) which would place blame on both parties (something like 70/30 in favour of the car over the motorbike if I remember correctly).
The case law relates to a vehicle waiting to turn right across one lane into a side road. A bike overtakes the stationary vehicle which turns as the bike is alongside.
Obviously the bike is carrying out a dangerous manouever (we assume the car is indicating), but as is previously posted - mirror, signal, manouver.
Personally, I pull up as close to the centre line as I can (or kerb for those cyclists) - if I'm stuck in traffic, they're all stuck in traffic!