It might be the Yellow-Browed Warbler, or the Hume's Leaf-Warbler. Both are usually found in Central Europe and in Asia. But, they will go as far as Great Britain and it isn't considered surprising to see them there. There's only one mark that I see that I think different, yours has a sort of dark band around the chest... and neither the Browed or Leaf warbler seems to have it. Well, here are some reference pics!
Yellow-Browed Warbler
Humes Leaf Warbler
And since I couldn't resist, another Humes Leaf Warbler
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