What course are you applying for?
I got into the Graphic Arts course this year, but declined them. I did a Foundation, but had a rubbish interview; really rushed and he didn't seem interested about the work in my portfolio and sketchbooks that he picked out when flicking through (I use flicking very literally). I was very surprised to see the offer after that experience, but then again I had AAA at A-level already so maybe that helped? Everyone else on my Foundation course who applied for the same course got in, and I don't think they all had ABB or higher, so I wouldn't worry. Of course, it might be different for Fine Art and Fashion/Textiles, if they're more competitive.
I don't think you'd necessarily need a Foundation for Winchester. It might help, but Winchester's BA programmes are very broad for the first year or so but then allow you to focus on a specialised area of study towards the end, so the first year might act a bit like a Foundation in introducing you to the different areas you can study. Having said that, they might favour a Foundation student over an A-level student. It might be worth going for it if you don't want to do Foundation, they certainly wouldn't rule you out for not having one.