The law on this is actually fairly simple. Lewisham have outlined the actions they take here:

http://www.lewisham.gov.uk/transportands...ements.htm

Have a look - but basically, unless there's a sign saying otherwise, parking partly or entirely on the footpath is naughty (though generally not criminal, so the good people of the Met won't be able to help) and a PCN will be issued by the council (unless it's on a Red Route - noted elsewhere on this site that those are the responsibility of TfL).

I'd be interested to hear what the result is if the offender on Dartmouth Road is the subject of a call from a concerned member of the public. I also wonder whether the cars on Devonshire Rd might provide a bumper payday for Lewisham's parking attendants...