The NHS flu programme is targeted at the people most likely to become seriously ill, which is a sensible use of public money and leaves a large group of working-age adults outside it. If you are healthy, under 65 and not a carer, you generally do not qualify, even though a week of flu costs you real money and you may live with someone who is vulnerable.
Private flu vaccination removes the eligibility test. £35, book when you want, no questions about whether you qualify. Adults and children, and the whole household in one appointment if that suits.
Worth being straight with you about price. High street pharmacies offer flu jabs for around £15 to £20, and if that is all you need, use them. Booking here makes sense if you want a specific appointment time, want children vaccinated in a clinical setting, want other vaccines or a health concern dealt with in the same visit, or would rather see a doctor than queue in a pharmacy.