I have to say these decimal feet being quoted were troubling me too, but if the differences are important then I'd say its far too tight to risk!
Yes I agree the other bridge seems tight too, and narrow but we know @blackrose goes through so it must be at least 12ft wide.
I suggest Ciara goes and spends a Saturday afternoon watching some boats go through, and watching how tight they seem to be and how big they seem compared to hers.
Also the full flow of the Kennet goes under High Bridge so the air draft in the bridge may well be 3" or 6" less on the upstream side than the downstream!