Have you seen any blank signs in London?

You need to put a spin on it.
How about photographing blank billboards and then putting up the images on empty billboards?*

• I haz MA
  
  

My major project last semester was photographing temporary things - signs, and notices mostly - that were supposed to be temporary but ended up being more permanent.
I was going to do big prints and stick them up like temporary signs themselves and rephotograph them. I ended up going for a cheaper method of presentation but kinda wish I had done the original idea now.