The UK vaccinated old people first. At least here in Germany we had the problem that AstraZeneca was only allowed to be used with people <65. That could be one explanation.
Not just immunocompromised. Group 6 was anyone <65yo with a clinical condition which made them more vulnerable. Nearly 7 million people were eligable for that. I imagine they have nearly been done by now, and at least half would have had AZ, so that's 3.5m young(ish) people that have had AZ used.
There are 441,498 staff working in care homes alone and nearly 90% of these were vaccinated in the first phase.