There are actually some unwalled 'open' urinals in Hyde Park (three urinals, iirc, are arranged in a triangle, all facing inward, so unless you peek around the corner you can't see other people while you're using it and unless you turn around no one can see your hampton)
There are 'pissoir' in the west-end of london that only come out during the night! they are the the three fold arrangement described by Lee - but they rise up from being hidden in the pavement about the time the pubs start closing. They appear as an extra-large manhole cover during the day. Took me a while to realise this, and I went through a period of great uncertainty cos I was certain that I had taken a leak at the bottom of villiers street and the next day there was no sign of any wc. there is one at the front of the theatre at cambridge circus too.