My GP clinic (i.e. the one of which the practice I am registered with is a part) is a PCN of nine practices. It doesn't work on postcodes.
If you're registered with any of those 9 practices, you will (eventually) get an invitation to make an appointment. Regardless of your postcode.
Since they finished the first 4 groups, they have not been getting much vaccine though. It's going to take them weeks to get through Group 6 at this rate, so going to a mass vaccination clinic was a no-brainer for me. Both are at large well-known sporting venues so plenty of space, and neither is particularly close to where I live, so none of that made any difference to my decision.