I don't know your area, but if you do manage to find a commercial tester to provide a meaningful certificate for your boiler, then I think that you will find the cost considerably more than club membership. In addition, the tester might be rather more demanding in terms of inspection requirements during the building process.
Clubs provide testing as a service to their members and the testers themselve volunteer their time.
My advice would be to pay the, probably modest, membership fee and join in club activities. There's much to gain.
Edited By Clive Brown 1 on 20/11/2017 20:31:35