I would argue that the UK is a uniquely bad place for wikipedia loves
monuments. Not only has it already been done directly:
http://www.imagesofengland.org.uk/
But geograph has also covered a lot of the ground. Repeatedly.
So what are the alternatives. If you want to insist on architecture
then everything listed in the Pevsner Architectural Guides is an
option. At least the stuff there has a reasonable chance of being
notable. Alternatively everything listed in the Defence of Britain
project
http://archaeologydataservice.ac.uk/archives/view/dob/
While I think photos of everything there exist they are not all online.
If people are prepared to move away from monuments options include
every single species native to the UK and underwater wrecks (which
have a higher challenge aspect). The species approach has the
advantage that we could also include videos.
--
geni