Hi; just did this very search for a feature about a month ago! There is an office block on Bishops Bridge Rd. W2 overlooking the railway that has black stairs. On Covent Garden Piazza next to the Church is a block of flats, again with black stairs. The rear of the Aldwych Hotel on Tavistock Street WC2 has some excellent ones. Shoreditch Town Hall has some good ones. We ended up using the ones at the old Queen Elizabeth Children’s Hospital in Hackney (through Locality). Also, I was told that the Westminster Industrial Estate in Woolwich might have some (through the Collective). Good Luck!
Andrew