For buildings look for the following creators on the DLS (to name but a few):
TafWEB
Railcentre
Domsarto
James73
Chrisaw
Bobsanders
Angelah
and even paulzmay
There are also plenty of European and American creators whose work fits in well in UK routes, for instance DMDrake, RW1HH and Cosey - just choose what looks like it might fit in a UK route.
As for locos, it helps to know what era you are looking for. Most free UK stuff is on the DLS.
Use www.tafweb.co.uk and http://magandy.co.uk/ukbl/
You don't need to try to connect to Auran for their stuff.
There might be others who host their own content but in my opinion they are the greatest.. (That is not say that Angelah et al aren't)