Certainly a post-privatisation sign - it's not in Rail Alphabet which is a giveaway. Looks more like Frutiger Medium to me.
The 'white on dark blue' signs at Clapham Junction date from about 2009/10, that's roughly when
SWT▸ started the changeover to that colour scheme for platform signage across their whole network, coinciding with the
DfT» 'stations report' of 2009 suggested it be rolled out as a national standard. (I don't think that has ever happened though.) Given how long these things take to happen it isn't surprising that they list destinations no longer served...
I think the house font is an SWT 'owned' design, but only based on a dim memory of a discussion elsewhere quite a few years ago.
Paul