Undertaking / filtering on left?

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?

Yeah it's mostly donkeys & hot-air-balloons in London.