As an existing O2 business customer the O2 website says that you can't keep your existing number if you get an Iphone.
So if you want to keep your number you would need to transfer (PAC code) your number out of O2 to say a PAYG sim on Orange etc, then transfer back in on a new contract.
My contract is up on November 11th with O2, might go PAYG or the 30 days notice Simplicity tariff and wait out until Iphone Rev B (3G, iChat AV).
As a contrast to the O2 Iphone prices, take look below at the cost of a 3G HSDPA modem for your MacBook/PowerBook.
I just got a 3G HSDPA USB modem with Three for free (normally £49, used old PAYG Three account discount) on a 18 month £15pm (inc Vat) tariff for 3GB per month with extra MB at £0.10, works well with my MacBook Pro. I signed up via Quidco.com for £55 cashback as well.
Three have 90percent 3G (384Kbps) coverage and are rolling out HSDPA 2.8/3.6Mbps.
Vodaphone 3G has 80percent coverage and have 3.6Mbps HSDPA all over the place with 7.2Mbps HSDPA rolling out in London.
Vodaphone is £25pm for 3GB per month plus cost of modem (free for 3.6Mbps USB) £49 for 7.2Mbps USB or Expresscard.
My old Vodaphone data contract was £43pm for 1GB fare usage!
Both Vodaphone and Three officially support the Mac and their 3G sims work fine in other 3G devices so an unlocked 3G Iphone would probably work.
If you only want Web, FTP, Ichat and Skype Three is fine, they do block some ports like Slingbox and VPNs are reputed not to work.
Vodaphone appears to restrict VoIP but works with VPNs, they also seamlessly switches between 3G/2G network, which is good on the train.