My understanding is that:-
All UK - Vodafone coverage is most complete.
London - "Three" (or is it "3") is possibly best.
Ask people what works at their location.
All of the providers now have coverage maps on their web site.
They often have seperate maps for voice and for mobile broadband.
They often specify, no coverage, outdoor only coverage, indoor and outdoor coverage.
Coverage maps:-
e.g.
UK Mobile Phone Signal Coverage Checker – Vodafone
Mobile Broadband: Finding your nearest transmitter | Mobile Fun Blog Has links to COVERAGE maps for all operators. The links are near the bottom of the page.
In general you type in a Postcode and they show you a map.
There is a Royal Mail address to postcode converter on their web site if you don't have a postcode. Postcodes are generally written by people with a single space, some web sites demand that you type no spaces and some the single space and of course many accept either
Postcode Finder | Royal Mail Ltd
Miss out building name if you don't have one - you usually won't.
Miss out building number if you don't have one but have a name - you usually will have a number.
Do get a 3 - 5 metre USB extension cable so that you can get the dongle say out the window or to some better location than stuck at the side of the laptop. I believe that most modern double glazing is coated with a metal film which is good at cutting off RF. For more that 5m you need "active" cables. I think you can use one normal "non-active" 5m cable and then three active ones at 5m each with the normal one connected directly to the computer. Maybe it's 1 + 4 active cables for 25m, not sure.
e.g.
£1 free shipping to UK. Given as example of exactly what you need.
Can be MUCH more if you have to get one from a shop in a hurry.
e.g.
Buy Belkin USB Extension Cable (1.8m) at Argos.co.uk - Your Online Shop for Computer cables, leads and plugs. 1.8m and £8.49 from a shop.