Europe didn't halt, they reallocated AZ to different age and priority groups.
The UK were the only ones taking this decision, months after teens in other countries had already been fully jabbed. Now, I'm not a vaccine expert and I can't say for a fact which of the decisions was the correct one.
But to me it seems madness that they actively chose to not vaccinate this particular group, whilst reopening schools with NO precautionary measures in them. Other countries had 3+ months' experience with the vaccines in teens, and had assessed the risk of Covid was still higher, so took appropriate measures on top.