I have just seen some 25mm quitled roof insulation, that says it
acheives 0.2 U value, its about the same price as 100mm kingspan,
which is also about the same U value.

Does it work ?

Its goona be much easier "materials handeling" than kingspan, and
thinner.

Neither product I have seen claims a BBA certificate. I am not sure a
BBA certificate means much, I have had products with certificates
fails, and the BBA arn't intereted ...

Rick