The full details for monthly and Pay & Go plans are on O2's site, but here's the rundown:
The 16GB iPhone 3G S will cost you anywhere from £184.23 (about $300US) to £0 on an 18-month contract. If you are willing to sign a 24-month contract, you'll pay either £87.11 (about $141US) for the 16GB iPhone 3G S or get it for free.
The 32GB iPhone 3G S will range between £274.23 (~$444US) and £96.89 (~$158US) for an 18-month contract and £175.19 (~$283) and £0 for a 24-month contract. Contract prices range between £29.38 (~$48US) and £73.41 (~$120US) for 18-month terms and £34.26 (~$55US) and £73.41 (~$120US) for 24-month terms. All plans include unlimited UK data and WiFi.
UK Pay & Go customers can get the 16GB iPhone 3G S for £440.40 (~$715) and the 32GB iPhone 3G S for £538.30 (~$874). If you purchase an iPhone 3G S with Pay & Go, you'll get a year of unlimited web and Wi-Fi for free.
As in the US, your upgrade options will depend on when your contract is up, though the O2 site says that you can usually upgrade during the last month of your term.
So UK readers, will you be getting an iPhone 3G S on June 19th? Let us know in the comments!