The one at Kent is no more, demolished maybe 10 or so years ago.
IIRC the bog standard GS&WR type was an arc-roofed shed made of corrugated iron, half sheeted down the sides, and maybe a bit wider than the Ratio kit shown. Perhaps there was something like a simple wooden platform inside to aid the carriage cleaners. But as mentioned above, most if not all of those were removed in or around the 30's.