Undertaking / filtering on left?

I'd rather be keeping a sensible distance from the inside of a car and be aware of what is behind me so that if a car does swerve, none of the above happens.

I don't know if things are any different in the big smoke but on my commute into leeds that is not often a possibility - what with cars parked in, pulling over into or driving in cycle lanes, or driving 2 up on a single carriage way. it's either over take, on the pavement, or wait and travel at the speed of the (5 mile long) traffic jam - what would be the point in cycling then?