As Scotman suggests, canvassing is normally covered by bye-laws (which can vary from one area to other) rather than by any specific national statute.
However many councils model their bye-laws upon standard formats, so there is often a good deal of conformity across local authority boundaries. Here, for example, is the relevant bye-law that applies in Tameside:
http://www.tameside.gov.uk/touting
You'll see that canvassing only contravenes the bye-law when it causes an obstruction or 'causes reasonable grounds for annoyance'. So (in Tameside at least) it's perfectly lawful to carry out canvassing in the street as long as you don't block the footpath or annoy people.
It's likely that other councils will adopt a similar approach, but the only way to be sure is to ask the council.
Chris