3UK are the best
Make sure you get the one plan which offers unlimited internet and tethering.
The drawback of 3UK is that they only rely on 3G/HSDPA networks and don't have a fallback 2G network.
They are also the cheapest amongst all iphone providers.
Biggest tip: When you buy a contract with 3, make sure u don't settle for refurb/refreshed iphone. They send you all the reject handsets which are previously used and faulty. Cheeky buggers 😀