As a military pilot (UK), I had always been told to make the decision at DH and that a subsequent dip during the go around was acceptable (and indeed expected, due to momentum). Looking at the ANO, Section 1 Part 5, para 47, it states that:
An aircraft to which this article applies, when making a descent to an aerodrome, shall not:
(a) continue an approach to landing at any aerodrome by flying below the relevant specified decision height; or
(b) descend below the relevant specified minimum descent height;
unless in either case from such height the specified visual reference for landing is established and is maintained.
(a) could be interpreted by some as you must not descend below DH period.
Personally, I think the part "continue the approach" means that as long as you have applied power for the missed approach at or before DH then you're OK.
However, its a bit of a shocker to find your understanding of a rule differs from the IRE's during a test (particularly if its a self-funded initial IR!!) - I feel for you.