So my o2 contract recently ended and when searching for deals I found giffgaff. GiffGaff uses the o2 network. All support is online. My old number was ported in in less than 24h.
It is technically 'pay as you go' - BUT you can buy "goodybags" which give you various deals. For example, the goodybag I buy is this:

Cost - £10/month
250 minutes
Unlimited texts
Unlimited internet (not subject to any fair use policy - but no tethering or ipad use)
Unlimited calls to other giffgaff users
Free calls to 0800 and other freephone numbers

iphone is supported, though they don't supply microsims, but I just cut my own down as I already had a microsim cutter.

I've found it flawless so far.

You can get cashback when ordering a sim card (eg £4 via TCB), but you won't get any free credit on it. If you order via another person's link both people get £5 free credit - THIS is my link and you will get £5 free credit for ordering a free sim card from it.
Note: if you want this for an iphone 4, just send me a PM with your name and address and I will send you a pre-cut microsim for free - you just then register it in your name on the normal way once received.

If you need more than 250 mins, then there are other goodybags available with increasing increments of call allowance included. As it is technically PAYG (even though you are buying monthly allowances) you are therefore not on a contract and are free to leave at any time! Nothing to lose.