Zacs is quite correct.
You cannot use valuations produced by surveyors, as they represent the market price for sales purposes. For properties in and around the south, a very significant part of the overall valuation is the value of the land which is never destroyed in a fire.
Building costs do vary across the country, but not by anything like the skewing of property prices.
I'm unsure what problem you are trying to solve, as insurance companies invariably quote very wide bands when they provide quotations for buildings insurance. Typical figures I have seen are "up to £500k" rebuilding cost with the next band being at £750k, or even £1m.
Spons is obtainable by anyone but it's price makes it professional use only. A lot of factors feed into producing it and it is far too detailed for what you are after.
The back of the monthly publication Housebuiding & Renovation magazine may still contain a table of typical building costs, by region. If you want a very rough and ready figure use £1400 per square metre of floor area (not footprint area).