For O2 - the one relevant for UK customers of iPhone:
MTS (Universal Mobile Telecommunications System) coverage map.
Please follow the steps given below to find if you have 3G coverage in your area:
1. Please click on the link
http://www.o2.co.uk/
2. When our website opens, please click on the 'Services' tab.
3. Now click on 'UK coverage' that appears on the left hand side of your screen (in the blue column).
4. The next page that opens, give you an option to check the 3G coverage in your area.
You can check this in any of the following ways:
a) By entering your postcode.
b) By entering your town/city
c) Your motorway junction.
d) Cell Site reference (if you know one)
e) By entering the name of the place.
If you need further details about O2, please visit our online Help Centre at:
http://www.o2.co.uk/help