Does anyone know if GRP is allowed on buildings above 18m in Tilbury UK?

You dont say what it is being attached to or how it is used. 18m sounds like you are  making a reference to regulation 7  - so appearing inapproved doc B clause 4. Note this restriction only applies to relevant buildings, such as dwellings and hospitals etc. I can well imagine that some sort of port buildings are not considered to be relevant buildings.

www.cwct.co.uk/.../government-ban-on-combustible-materials-in-external-walls-of-high-rise-buildings

The final  authority on such matters will be the local authority building control department.
Mike