Thankyou for all the contributions. Yes, it is very complicated and there is lots to consider with the business side and of course meeting regulations and safety standards. I am aware and proceeding with most of those points, there was just a couple I was unaware of, Thankyou for highlighting them.
So to summarise the answer to my original question-
I take it that I would have to earn over £85k (which I wouldn't believe I
would even at best estimations!) So therefore I would not have to pay the VAT for change of liveaboard. *
* Im going to triple check this with HMRC.
Thankyou