Go to Luxembourg. I was amazed how close the local birdlife allowed people to get without spooking. I could have filled the frame with sparrow using my 50mm!
Yep, the U.K. is tough place to get close to birds... birds are far more tolerant in many countries.
Back to the o.p.
All birds have a circle of fear... a certain distance around them that they'll accept your presence, but with a bit of care you can reduce that distance.
Basics, keep as low to the ground as possible... i.e. reduce your visible profile, do not approach them directly but zig-zag your approach. Stop fequently and let the bird get used to your presence at that distance (good opportunity to take some shots, as the bird could fly off with your next footstep). Try not to have too much direct eye contact with the bird. Do not do anything suddenly.
Camo is debatable, but you will certainly be better off wearing clothing with a break-up pattern to make your profile less distinct and to give less impression of movement by you... dull tones work best. Bright patches of skin can cause problems, so a piece of scrim over your head is not as silly as it sounds (and you can use a camera throug it), hands can be covered in gloves.
Keep quiet.
Birds are fairly obvious when they are unhappy and about to fly off, they ruffle their feathers and shift uneasily on their perch... if you see this, stop.
it takes a lot of practise, but you can get very close at times... but different species, even different birds of the same species react differently.
cheers,
Andy
