This is primarily due to a large fleet being built in Britain though - how much of that fleet was the RN planning to build using Cockatoo Island and Canadian yards, let alone in NZL and SAF?
In the vanilla game, the Achilles starts off as an RN ship, so that's already covered off

. Also, while I don't know the figures, I imagine that the amount of money being given to Britain for Singapore wasn't the equivalent of 33 per cent of the cost of maintaining and building the RAN (happy to be proved wrong as always though

), and the money was provided for the construction of the base wasn't it, rather than for construction of new naval assets to be stationed there?
My key point here was given the amount of NIC available to minors that wasn't there historically. Any of the dominions can far out-build what they did historically with their starting NIC alone (historically, NZ and SAF didn't build a single ship to HoI4 standards, and Canada and Australia only built four destroyers apiece, IIRC (going from memory though, so may have forgotten something)). Even if Britain only gets 25 per cent, it's 25 per cent of a significantly larger slice of NIC than it should have access to even if it was allowed to have 100 per cent of the historical NIC of the dominions.
As an aside, what RN construction of HoI4-level ships was done in Australian and Canadian yards? I'm not aware of any (all the HoI4-level ships Australia and Canada had at the start of the game (aside from HMAS Adelaide, built well before 1936) were built in Britain, and while Britain did get Australia's seaplane carrier, which was built in Australia, this isn't within the scope of the game).