http://www.sitefinder.ofcom.org.uk/search is a useful site for 3G router installations. When you type in the postcode it shows a map with all of the nearest mobile operators masts highlighted.
The other good aspect
to knowing where the masts are is that for some installations where the signal
is poor from all operators, an external, directional antenna can
help and it is useful to know where to point it for the strongest signal. The higher the antenna can be positioned, the better.
For Three UK you can also use http://www.three.co.uk/support/coverage and remember to check the mobile broadband button and for Vodafone use http://www.vodafone.co.uk/vodafone-uk/personal-coverage/index.htm and check the button for internet and email