Ady, I think that your GP did a good job with that range of tests.

Elevated anti-thyroid peroxidase antibody levels are present in >90% of people with Hashimoto disease.

What might be happening (among other things I am an immunologist (PhD) but not a clinical immunologist (no medical qualifications) is that you are in the early stages of Hashimoto disease with no signs yet of compromised thyroid function - T3, T4 and TSH are all fine.

Alternatively, maybe the technician who ran your anti-thyroid peroxidase antibody test stuffed up (it happens...).

Next time you see your GP it might be an idea if you asked him/her to arrange a re-test to check this out. If the high value is confirmed then ask your GP how often your thyroid function should be checked - usually Hashimoto disease can be managed well with synthetic T4.