Call me a cynic, but I think the get-out you describe is far from unlikely, although I know very little about the detail of planning regulations.
It now appears to be open season on public access to any new facilities or adaptation of space in the SE1 area, and I don't think that the GLA (or at least its current administration) would really want it any other way.
I think Johnson's defence of the Underpass skate-park was simply prompted by its potential to attract more tourists (and thus commercial revenue) rather than due to any concern to preserve the public use of open space.
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