I'm nearly 100% sure that the test has to be done at a test center. Reasons are that you use their IT equipment which sets up the individual tests and that there is an adjudicator present to prevent cheating.
You would need to run the earth from the consumer unit to the unearthed sockets. Are the sockets the old type without the connection for the earth? If so you are going to have to replace the socket outlets also.
Surely there still has to be some sort of a motorised valve to close the heating circuit. If not then you will always have heating when the boiler fires.
In your scenario the highest neutral current possible is 100A. That’s if 1 phase is fully loaded ant the other 2 have no load. Add a load to the other 2 phases and the neutral current reduces.
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