I don’t know if she’d seen them before, but the platforms don’t make sense because she mixed it up with Euston station.
Interestingly, in an interview with the BBC in 2001 JK Rowling admits confusing Euston station with Kings Cross Station. “I wrote Platform 9 3/4 when I was living in Manchester, and I wrongly visualised the platforms, and I was actually thinking of Euston, so anyone who's actually been to the real platforms 9 and 10 in King's Cross will realise they don't bear a great resemblance to the platforms 9 and 10 as described in the book. So that's just me coming clean, there. I was in Manchester; I couldn't check.” In fact, even at Euston station platforms 9 and 10 are also adjacent meaning that once again, there would be no room for a magical brick wall!
That’s from a Historic UK article about Platform 9 3/4; the full interview can be read on Accio Quote.
The article also explains that (at least for Chamber of Secrets), they used platforms 4 and 5 instead of the real 9 and 10.
The HP Lexicon has a Gallery of London Location, which includes pictures from the relevant platforms in King’s Cross and Euston.
This is Platforms 4 and 5 at Euston, where the barrier scene is filmed in the films. However, the pillars in the films are clearly wider than this, and don’t have as much gubbins climbing up the side.
You already highlighted the lack of barrier at King’s Cross, but as the quote points out, this is also missing at Euston. It’s somewhat obscured, but you can see the two platforms are adjacent, and thus there isn’t a large barrier between them.