The only issue i have with low speed limits, be they 20 or 30mph, is where it is not clear what the limit is. If it is clear, abide by it and there will be no problems. Given that we rely on our licenses to do our job, it is our own responsibility to make sure we are not speeding. As a bus driver, doing late runs into Aberdeen, i would often use the cruise control on the bus to maintain the 30mph, otherwise it is remarkably easy to let your speed creep up without realizing.
With regards to bus lanes, i cant speak for anywhere else, but in Aberdeen, they are for bus and taxi use only during rush hour in the morning and afternoon (the times are clearly marked), otherwise, anyone can use them.