Lots of walk-ins are giving you your booster even if you’re technically not eligible yet. For example, locally to me, they are accepting ppl who are over 18 as long as it’s at least 3 months since their last jab.
Check out Reddit for more info local to you.
Just in case there are ppl who want it and can’t book. (gp’s and pharmacies are different, they can’t give it early but walk ins are doing the business)