I have both a Galaxy S3 and an iPhone 5. I don't see the point of selling the S3 for an iPhone 4S unless you're seriously strapped for cash and MUST have an iOS device. Get the iPhone 5 or keep the S3. Don't bother with the 4S.
the 4S is an excellent phone but if you've got 330 quid now to spend might as well go the extra mile and get the newest phone and dont regret it down the line when and if you want the IP 5 you'll have to pay full price.
It didn't feel terribly different going from a galaxy note to an iP5 but going back (to look up a contact today) was WEIRD. I'd go iPhone 5 though, it's out so you might as well get it to stay up to date longer. Two things make me say this: larger screen and ridiculously faster specs. Everything about it is double over the 4S... Just a guess, but I'd say it won't be long before developers are really taking advantage of that.
I upgraded from a 4S to a 5 and I can tell you that besides the obvious cosmetic differences (thiner, longer, lighter, aluminum back) there is a HUGE difference in speed and the display is noticeably more vivid.
Get the 5...you won't regret it.
I sold my S3 to buy an Iphone 5 as well. Very happy with it! I love the build quiality and the fact that I can operate the phone with just one hand. Sure the S3 screen is great when at home sitting on your couch or something, but on the move the Iphone is more handy to me.
4s is also great but the 5 is really fast, will hold it's value better, if you can miss the 199 pounds...go for it. If you can use the 199 pounds really good for something else, the 4s is still a great phone. So get the 5 unless you have a really good destination for your 199 quid now.