It totally depends upon how you intend to use the phone. When are you likely to make calls, will they be to mobiles or land lines, do you want text messaging?
Your best bet is probably to go somewhere like the Carphone Warehouse and explain your requirements. They'll be able to advise accordingly.
The best deal I know of at the moment is in orange shops...200 x network calls, 500 txts a month �30...and you can get a free phone if your phone is in working order....you dont even need to take the manual or charger
Have a look at o2 online cos they're usually quite good. It totally depends on your usage though. One of their current offers is 500 free texts anytime, 1000 free off peak minutes (o2 and landlines), some free WAP time and all for �20 a month. Oh yeah, and you can get a free phone too. All bills are sent to your email address and you pay by direct debit every month.
vix2 - I've been an o2 online customer for the past 3 years and have never had to call customer services. You can email them with any problems you have and in my experience, they've responded within 24 hours of my query and solved it. But all the same, I'm sure they make a fortune out of people who do call them!!