I'm just curious about this and wondered what was considered rude. If you are visiting friends who have obvious space in their drive do you park in it or on the road?
Personally I wouldn't park in it if I knew the space was about to be used by their DP coming home or similar but if I knew that the space was available for the duration of my visit and the alternative was to block up the street or pay for a ticket then I'd assume it was ok, and ask whoever I was visiting if it was ok.
Similarly I always ask visitors to park in the drive. We have a lot of space but the road we are on is very narrow and parking in it causes congestion so I feel guilty if visitors park in it when we have plenty space.