The general consensus will be the Collins guide and indeed it is a good guide but I don't think its very good for a beginner or even for a not so beginner.
For a start its not a UK only guide, in fact its not even a Europe only guide.
The Collins to quote covers "All bird species which breed or regularly occur in Europe, North Africa north of 30 N, and Israel, Palestine,Jordan, Lebanon, Syria, Turkey, Armenia and Azerbaijan, that is in a large part of the Middle East. Also included are the Canary islands, Madeira and the Sinai peninsula."
All great if you travel a lot or are hoping to see lots of vagrants.
As an example if you're looking for your first Dartford warbler and look it up in Collins you find on the same page 5 species which look fairly similar. All except the one you're looking for are only found in north Africa or some islands in the Med. Why do we need the confusion. I wish they'd bring out the Collins with the excellent illustrations with a UK focus.
The large coverage brings up problems which I think could well have been avoided in a UK version.
1.) Often 4 or 5 species are listed per page with the inevitable small size, the font size of the type is really small.
2.) Its not very pocketable
3.) The paperback version doesn't open flat very easily.
I think a better option is the Knightley, Madge Pocket guide to birds of britain and NW Europe
This is much more pocketable and has an easier to open binding with a harder cover. It covers less species often only 1 or 2 to a page which means its much easier on the eye. The illustrations are not so good though. It gives population estimates for the Uk which I find useful sometimes although I realise they are probably out of date. Also gives measurements in metric and imperial which I find easier to understand the collins only has metric and I still don't have the same grasp of them as I do english units.
http://www.amazon.co.uk/Pocket-Brit...0434846?ie=UTF8&s=books&qid=1175362045&sr=8-1
The answer is probably to get both but I think for the guide that you will want as a beginner the Collins still has a way to go. I wish they'd follow the US example of the Sibley where theres the coffee table version and the slimmed down field guide versions.
Sometimes the collins can be found quite cheap, a while ago bol.com was selling them at 3 pounds each, so always worth looking around but really for what you want I'd recommend the Knightley Madge pocket guide.