Thanks for posting this query. The EX-A document is not something we recognise as a UK document. Looking on the internet we can find references to an EX-A document as a Customs entry for export outside of Europe, which in the UK we call an NES entry and the printed document is an EAD (Export Accompanying Document).
Do you think this might be what your shipping company needs? It wouldn't be something that many Chambers of Commerce offer but you can either complete the export NES entry yourself via the HMRC website, use an agent in the UK to declare it on your behalf or we provide software to enable you to complete the entry in a user friendly format.
It wasn't as prominent but some websites suggested the UK version of an EX-A was the EUR1, a preference document which can offer a lower rate of import duty for certain goods to certain countries. That is a service Chambers of Commerce offer and you can apply online via e-z Cert.
We're happy to help further if required.