If this is the case then you should get a PAYG iPhone of whatever size you think will suit you (16GB sounds enough for your needs) and then get a Simplicity Sim Card.
See here:
http://shop.o2.co.uk/home
For less than £20 per month (and a rolling month by month contract) you will get 600 minutes talk time, 1200 texts, and you can choose the free Unlimited Web Bolt-On.
Superb value for money without the commitment of a lengthy contract, and everything will work on the iPhone except Visual Voicemail and Call Merging (neither of which are such a big deal).
🙂