Why do some areas win more than others?
Only postcodes entered into the draw (and verified) can win.
Each postcode entry has the exact same chance of being selected in a draw.
If one postcode is entered more than once it is more likely to be selected (like names in a hat).
If a certain postcode area (eg. BA1) appears to not win often that is because there aren't many entries for that area. If a postcode area seems to win more often it must mean that there are more entries from that postcode area.
You can look up how many entries there are from a postcode here: Entries Search
You can also look up past winning postcodes (or partial postcodes) here: Past Winners
You should find the ratio between entries and winners is roughly the same for different postcodes/areas.
There is no geographical bias in the selection procedure. The computer doesn't know (or care) where you live!