Firstly, I'm in the UK, live in London, and I've had an iPhone working on O2 since it could be unlocked. Some thoughts...
£270? UK users are getting ripped off again.
The contract prices are a joke. I'm currently with O2, and on a cashback plan, pay £3.35 a month (12 month contract) for 400 minutes/200 texts - even better deals are available now, such as £0 ! a month for 800 minutes/500 texts. Who in their right mind would sign up to one of the rip-off O2 packages, when "unlimited" data seems to be very limited indeed - 1400 pages - what??
This is typical rip-off Britain, and I know plenty of Apple fanboys will sign up and buy this phone anyway. But if they had any sense, they'd buy a phone in the USA, unlock it, and find a better deal with another provider. If rumours of what O2 had to pay for the iPhone deal are true, it's clear they're passing the cost on to their consumers.
As for who has the best coverage between O2/Vodaphone/T-Mobile etc, this is a pretty moot argument. Across the UK, this varies massively. e.g. In Oxford, O2 and Vodaphone are great, in Winchester, Orange is great (and you can barely get O2 coverage), and in London, various networks suck in different areas. I think the main point to be made is that T-Mobile offer the fantastic web-n-walk package at only £7.50