I've got a job we're about to look at which has some cladded roof repairs to do. It's a warehouse built in 2005 in the heart of the Midlands and the eaves of the sheeting going into the valley gutters is showing signs of delamination and leaks on rooflight joints etc. Should get the quote and scope in the next week.
You're right about the coatings on the external steel, it should be classed these days as corrosion class C4
C4 High Urban and industrial atmospheres with moderate sulphur dioxide pollution and/or coastal areas with low salinity.
It'll definitely be ready for some remedial treatment after 25 years