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The only issue might be that the first booster (3rd shot) was a half dose of Moderna which was proven to work well as top up to 2x AZ or Mod/PHZ, not sure what the pfizer dose was.
So there is maybe an argument for a full sized shot of Modena/Pfizer for you.
But if you have had covid before then a current booster (bivalent) will top you up.
I would speak to either a GP or go to one of the larger vaccination centres for your area like the ones at Kings/Guys/StThomas’s (these are SE london health authority) I know the bigger centres offer first/second/booster doses but smaller pharmacies didn’t last time i looked.
I am not a qualified/trained health professional
I've had three doses of one of the Chinese vaccines but would like to get a western vaccine once I'm in the UK - can you choose which one you want?