Flu jab - random question

Does anyone know what the criteria is for a free flu jab?

I finished cancer treatment (surgery, chemo, bracytherapy) nearly two years ago and that first winter was given a flu jab at my GP practice as I was still immune compromised.

Last year I didn't bother and got away with it, but I've already had three bouts of flu-like viral infections this year and numerous colds.

I've resorted to mega dose vitamin C to try to boost my immune system but just feel like I'm a sitting duck for the flu virus.

Is it a fixed criteria, or do providers make their own judgement?