SOLVED. Horror apocalyptic zombie take over in England [s]

[Andy Phillips]

Do you know anything else at all? This is a complete stab in the dark, but maybe Autumn. The characters aren't young though. A better guess is probably Blood Crazy. Not exactly zombies, but it has that feel to it.

[Lobstergirl, au gratin]

Amy, Autumn was originally published in 2001. Andy Phillips had linked to an earlier edition.

[Jen]

Could it be The Enemy series by Charlie Higson?

A sickness has afflicted everyone over the age of 16; anyone who is a “grown-up” has become a decomposing, brainless creature that survives by feeding on children. The children and teens have barricaded themselves in fortified buildings, fighting off attacks from the grown-ups who travel in packs, like hungry dogs. Before long, the young survivors are promised a safe haven in Buckingham Palace and make their way to it, crossing London on a perilous journey that will test them in harrowing ways. But their fight to stay alive is far from over-the threat from within is as real as the one on the outside.

[Andy Phillips]

Great! Glad you found it. I enjoyed it despite a couple of aspects that I never really understood.