On Friday afternoon, I also popped in to Blackwell's bookshop at 100 Charing Cross Road - see this location map
Blackwell's staff were aware of their signing promotion, but thought that it was "in September" (actually on Tuesday 28th August at 7pm at the TUC conference centre in Great Russell Street, near the British Museum), and that they might advertise this on their website "soon". Again, there was no Penguin or Blackwell promotional material on display.
However, Blackwell's did have a couple of copies of Spook Country on their computerised inventory. A helpful member of staff searched through three boxes of newly delivered books, to dig one out for me !
N.B. the UK publication date is August 2nd, ahead of the US one on August 7th.