Odd. I do have allergies but I'm not younger than my children
I guess the message is being given for the same reason then.
You are eligible for the vaccine under the national system and therefore entitled to it.
You can only have one of the vaccines (usually this is because you are 16 or 17 and can only have Pfizer) so the GP has to arrange it (even if their personal opinion is that you are not eligible - it is too late for that, the national system already says you are eligible).
In your case, if you are over 18 and have allergies, it maybe that the system can also identify that you can only have AZ due to allergies.
If the GP is refusing to organise a vaccine to which you are entitled in the national system, you can contact the CCG or other patient complaints group.
Alternatively you will have to wait for your age group to be eligible, if you allow the GP to block your access to the vaccine. Typically it will be admin staff who do this. Have you spoken to an actual GP?