I would have to disagree with this as a rider of 5' 4" to 5' 7" would best suit a frame of about 50 to 52cm and no bigger, but this would of course depend on the persons inside leg measurement. A frame of 54cm would generally be for a rider of about 5' 10" but this guy may have just had the saddle a little low. Also the saddle in the picture looks to be very thick from top to bottom and this would also affect the seatpin extension. Either way it's irrelevant as the frame is 54cm so you will either think it fits you or you won't - simples!!!
I would question that.
I ride a 56 non compact and I have almost 4" of seat post on show, choosing a 54 at 5'10" may well give you an unacceptably short top tube and push a shorter seat post beyond it's minimum insertion point, you would have to stick a MTB post in to stop it fracturing the seat tube lug and clamping collar. If I rode that size I would run a good chance of striking the bars with my knee climbing out of the saddle as well.
Stumpy legs you might get away with it, but thats' pushing it.
The bike seems nice, around the £125 mark is a good price point, I suspect it's a Raleigh/Tube Industries frame underneath so it might not contain as much butted tubing or be quite as light as you would expect for a "531" frame, age wise I would date the bike from the late 80's, I have seen nigh on identical rides with Raleigh Holdsworth and various shop names on the side
It's still perfectly serviceable though.