Why is it such a 'total pain' though? I ask as a non-driver, but an active user of roads for cycling and urban routes for walking/cycling.
Personally, I'd say 20mph on urban routes is not a major issue. Besides, there's plenty of urban routes that are more than 20. Take for example the A483 out of Swansea, I'd personally count it as an urban route until it reaches Fforestfach Retail Park given it's still within the city. That's definitely more than 20! I also didn't see much at all in the way of any 20mph urban roads on my cycling around Swansea a few weeks ago. Places like Kingsway, absolutely that should be 20mph with the many bus stops, pedestrian crossings and junctions. A lot of the city is not 20mph!
I would however agree with
@birchesgreen that basic signage is beyond the realms of maintenance in my city, and don't start me on the unreliability of those electronic speed boards in villages etc which show the driver how fast they're going.