Hi
If your buying them outright then you will have to acquire them from the Apple Store as O2 only supply them on the Refresh Tariff right now.
Their is an online reservation link for Apple so that you can reserve at just after midnight and collect the following day.
http://www.apple.com/uk/retail/iphone/As for tariffs I would check the individual sim only ones here
https://www.o2.co.uk/shop/all-tariffs - their are 30 days and 12 month options. You get more for your money with the 12 month options.
Or look at the Refresh ones too. Your not locked in as you can pay off the handset plan anytime you want ,and carry on with the airtime plan or end it and walk away from the contract.
Be aware that O2 don't typically unlock the new iPhones for approx six months after launch. The iPhone's bought directly from the Apple Store are fully unlocked.
Business tariffs are here
http://www.o2.co.uk/business/products-and-services/mobile-and-tablets/mobile/tariffsI would comparison them and see which is going to suit your individual needs.
Let us know how you get on.