There are walk-in clinics available for those who are NOT registered with a GP https://www.nhs.uk/conditions/coronavirus-covid-19/coronavirus-vaccination/find-a-walk-in-coronavirus-covid-19-vaccination-site/
Since I am over 50 (yes, I know you don't believe it) you can book in advance of the 6 month "milestone" anniversary of when you had your 2nd jab.
But you still have to wait for the actual milestone to actually get the booster jab.
At the moment there was a very large number of local slots to choose from. I was able to book a local chemist at a time that suits me on the actual 6 month anniversary day.
This may not last so get booking.