Hi Moog, used to live in SW18, and tried a respo mask once, from memory, I used this only on one ride, as the increase in effort to breath over not having it on was too great.
The respo is really the only option if you must have a mask in the UK, anything else will be either too industrial (3M types), or too light weight (painters)
Would suggest checking out how other many riders use them, as I can't remember seen many, if any.
The polution you are going to get will be less than if you were sitting in a sealed up car, and all the trucks and busses there have to be Euro III compliant, so should be pretty clean; just don't sit behind an exhaust!
jimc101