FAA Airworthiness Circular AC43-4A pretty much sums it up in figure 4-20 when they put the whole of the UK at moderate to severe corrosion severity zone...
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%2043-4A/$FILE/AC%2043-4a%20.pdf