Please don’t book your second dose at a Vaccination centre if your first was at your GP. You should go back to the same place you had your first jab - there’s no guarantee they’ll have the same type of vaccine at the Vaccination Centre when you go (both GPs and the VCs have had Pfizer and AstraZeneca in the past and there’s no guarantees of what they’ll have on the day of your second jab).
If you had your first with the GP, and don’t yet have your second appointment booked, they will definitely be in touch with you to book you in, and will know that they will have the right vaccine available on the day.