FFF - Agree with what you're saying for QFE.
Empirical Method
What you then need to do is know the field altitude AMSL. Divide that by 30 and this will give you the number in HPa that you need to subtract from QFE to give you the field QNH (if you're at a field with -ve altitude above AMSL, then add
).
For the regional QNH, you need to obtain the forecast QNH for the region.
'Proper' Method
Get on the radio and contact London / Scottish Information or your nearest ATC and they'll tell you Regional QNH.
HTH.
[Edited by Circuit Basher because he made a complete a*$e of himself on the first attempt!!]