When considering motorcycle accident liability, there are 3 types of undertaking:
Any undertaking should be done with care. If you were weaving in and out of traffic when you had an accident you are likely to be found responsible as well as being prosecuted for careless or dangerous driving. Where filtering at an appropriate speed the other driver is likely to be found at fault. If you were travelling between lines of traffic a lot will depend on what the other traffic was doing, whether you were passing each vehicle individually and how fast you were going.
Undertaking is usually dealt with on the same basis as filtering unless it was clear that the motorcycle should never have been there. This does not mean a motorcyclist illegally travelling in the bus lane is automatically at fault. What matters is the reasonable expectation of both parties of the potential dangers and how they adjusted their driving accordingly.
Each case is likely to depend on its specific facts. However, any conviction for either careless or dangerous driving will go a long way to reducing any responsibility of the other side, so it is always worthwhile obtaining legal advice at an early stage.