In message , Basil Jet
writes

The majority of buildings in London with this sort of appearance have
worms of something, presumably stucco rather than concrete, stuck on
top of flat blocks of something, presumably stone rather than concrete.

In the case of stone, vermiculated rustication is created by roughly
hewing the surface of the blocks - nothing is applied to the surface -
as in the following shot of Somerset House:

http://www.flickr.com/photos/barbararich/165208828/
--
Paul Terry