I was told by NCP, who run the traffic wardens here in Brighton, that window cleaners who are working within sight of their work vehicle are not supposed to get parking tickets if parked on single or double yellow lines and that rule is, or should be, the same all over the country. The only time that rule doesn't apply is if there is single or double stripes painted on the kerb stones and the plate on the post states 'no loading or unloading', etc.
If you're worried, just take something back to the van and then take something out every 10 minutes as then it's classed as loading or unloading.