At the time of writing there are only three mobile phone operators in the UK that have the iPhone 4S on PAYG; Tesco Mobile, O2 and T-Mobile have yet to confirm if and when they will be stocking the iPhone 4S on such a plan. As for the rest, Vodafone, Orange and Three Mobile, here's the latest update.
All but the 64GB black iPhone 4S are out of stock at Orange online. Orange sells the 16GB model for £490, the 32GB for £580 and the 64GB for £660. In all cases, you will have to buy at least a £10 topup (and you save £10 off the price of the phone when you top up £50).
Three Mobile sells the iPhone 4S for £500, the 32GB for £600 and the 64GB model for £700 and seems to have plenty of stock around (with delivery dates expected as early as the 26th of October 2011). You will have to buy a £15 topup though.
Vodafone on the other hand is capacity constrained with no stocks of the iPhone 4S 16GB (either colour at £499). There's plenty of stock of the 32GB model (which costs £590) and limited availability of the 64GB model (which costs £680). As with the others, you will have to top up a minimum of £10 in order to use the phone.
It's worth bearing in mind that Apple charges £499, £599 and £699 for the 16GB, 32GB and 64GB iPhone 4S respectively. It means that, apart from Three Mobile, the two other mobile phone operators (Vodafone and Orange) are selling the iPhone 4S cheaper than Apple's own price (especially when it comes to the higher capacity models).