Postcode letters are based on where the mail is sorted not where the mail is delivered. You need to remember the postcode is only for the PO benefit. But like social security numbers in the USA, they are used as a hash for many address lookups and that’s just so wrong!
There are a few gotchas like the Wirral having a Chester postcode, but its mail is now sorted in Chester not Liverpool. (The best thing that ever happened because when my postcode changed from L to CH my car and house insurance dropped even though I did move area!)
Northern Ireland is easy, everything is BT but you can count it 6 times. Likewise the Scottish codes can be counted 3 times. This is because the sorting office density is so low there are few postcodes to cover a huge area. If in doubt use the code for the nearest town. Anyway, there are so many only resources that you can use it should only take moments to find out any postcode.
Yes the VHF contest committee know all about postcodes and the non-sequitors that crop up. But they don’t change too often. They were brought in as county multipliers were dropped due to the changes in local government boundaries in the 90’s. In fact since I stopped actively contesting in 1999 there have been many significant boundary changes but few postcode changes in comparison. So unless you can think of some other piece of info that gives geographic info that can be used a multiplier then we’re stuck with them!
Best of luck in the contest and with the activations.