I enquired about the public realm function of central London TfL roads, considering that the professional highway planning guide, the Manual for Streets (2010) calls roads with an important public realm function - ie. most in central London - 'streets' for planning purposes.
Transport for London gave me this advice:
"The guidance published by TfL on streetscape principles (“Streetscape Guidance 2009: A guide to better London streets” , link) applies to any public highway managed by TfL (TfL red routes such as Euston Road) and it therefore goes beyond the traditional classification between roads and streets.
Documents published by London Borough of Camden and the Mayor of London (Local Development Framework, Unitary Development Plan, London Borough of Islington and Mayor’s London Plan) guide the development of King's Cross Central including King’s Cross square. I would advise you to contact London Borough of Camden to find out about specific design principles applied when King’s Cross square planning application was reviewed."
I found that the Mayor of London in 2009 included King's Cross Square as one of "London's Great Spaces" but at that stage the street (A503) was not part of the square, nor was this clear in Camden's LDF, its UDP or its SPD. The Planning Applications (2010/3152/P, 2011/0019/P, 2011/1961/P, and 2011/4782/P) for King's Cross Square (discussed in the news) are on Camden's planning applications website.