If memory serves me right the issue is not number of pax ( comercial) for VAT exemption but is GRT and that is 15 tonnes. This, on the basis of VAT excempt for commercial vessels, puts you out of the frame.
The CI are not part of the EU so harmonisation of VAT numbers does not allow for one business to another zero rating. We are aware of this as we export to the CI. I have all the relevant PDF files in the office and if you remind me I will get the office to fwd to you.
However if you register ( by coding and VAT registration) as a UK passenger vessel then your worst case is paying the VAT on import and then claiming back under the business. Do note though that the vessel must be part of the business and not just a hidden pleasure.... maybe your corporate computer days would count ?!! I am unsure as to Duty on the imports but it is well worth looking into as in some cases it can be hard and is not reclaimable.
Also ensure that the builders certificate is acceptable in the UK along with RCD documenation ( in full) and if a coded vessel expect a tube test certificate( detailed) and a SOLAS equivalence declaration and finaly some independant audit on build in case it goes wrong. A full and detailed stability booklet(though rare with most builders) also helps. An ISO test certificate on the fuel tank is also to be expected as is some evidence of drop tests on the hull. I know that Delta, Humber/Quinquari and Halmatic do all this but worth checking on others as ultimately if coding fails then so does your VAT test.
I think you may have become confused on the 12 pax under the VAT zero rating for passenger charges on board. This is the marine voyages bit that also alows the duty claim but is nothing to do with import VAT and duty.
As an aside if you still need the use of a 6 wheel trailer then there are now a few here FOC for a few days if you require.