Taking East Anglia as £100, here are some notional building costs for other parts of the country as compared to that (notional) £100:
Northern Ireland £76
Wales £93
Northern England £97
Scotland £100
Midands £102
Southwest £106
Southeast £114
Outer London £120
In other words, for every £100 you would expect to spend on a self-build in East Anglia, you can expect to spend £76 in Northern Ireland, and so on.