The first thing that struck me was that you already fund a big chunk of household costs:
I pay for all the non utilities such as clothes/uniform/ pet insurance/dental bills/Christmas and birthday presents/ trips and school expenses etc
Those ought to be collated with the other house/utility costs/groceries/holidays etc. There are then two ways of dealing with it. Either you then pay proportionately in line with income and keep what is left for your own personal saving/spending. Or you both pool ALL of your money, pay all household/family costs from that and either split or at least have joint free access to what is left as disposable.
I expect if he paid a proportionate share of everything (88.5% though I appreciate that will need some adjustment for higher rate tax but roughly) he would actually be paying more than he is at the moment.
The disposable income is also a huge issue though. Let me guess, he doesn't share his with you, at least not freely? And your "disposable" income from your £17k salary also ends up being spent on treats for kids and not just you? There are two problems with that. Firstly, his standard of living is much higher than that of his own wife. Secondly, he gets a huge benefit from your non-income generating contributions: primary parenting for your joint children, nice meals cooked, housework and chores done. I'm guessing the mental load and care of the pets falls to you too. He has even told you he "won't" do that stuff as he doesn't want to! The quid pro quo ought to be that in return he shares his disposable income, rather than has a go at your for not paying a bigger share of the bills.
I am not someone hugely in favour of joint accounts. (We have one for household stuff but keep our own disposable income separate but we both work FT and there is much less disparity between our incomes.) But where there is a huge discrepancy and one partner is making a huge non-income contribution by agreement, the total income really ought to be more fairly shared.
Would you be better off if you divorced his arse? You'd probably get over 50% of the assets and he'd have to pay you over £20k a year in child maintenance. This isn't a "LTB", I am just saying it is a point you could make (if it is a good one haha) to show it is him being unreasonable, not you.