Help me work out if this is fair?

Of course he knows why.
So he's a lazy arse at home and a liar to boot.
OP carried him so he could focus on the business whilst not taking much from it.
Turns out he worked all hours, all those years for very little whilst she did it all.
No wonder she is pissed off.
Yanbu OP, not least because he is a bullshitter.
He knows why they wanted him out.
Probably because he was a lazy arse at work too.

How convenient that he works part time now the kids have grown up. You both should be working FT and saving for your old age. Living frugally when you're older is not fun!
Could you retrain or study while working so you can earn more?

Oh he knows!

I have a similar story to yours.
My dh does all the food shopping, cooking and washing, vacuums too.

Don't let this eat into your feelings for your husband and potentially ruin your relationship.
You seem very resentful OP and I am not convinced that they are justified. You are fortunate that you worked part time until your children were teenagers and it's not your husband who chose to leave his business.
The inheritance he received and money from the business paid off the mortgage which is a very privileged position to be in.
Please try and change your perspective on this, for everyone's sake

Surely the sexism here is you and her husband. When the OP was part time she was also looking after young children and doing the housework and gardening. Now the husband is working part time he's not.

Of course he knows why his business partners wanted him out, he just doesn’t want to tell you.
Neither of you have much in the way of a pension so if you want to stay together, you both need to prioritise this. Bickering and resentment won’t get you any closer to your goal of a comfortable retirement.

You have a household income of 48k, the mortgage is paid off, and you have years to go before you retire. Why on earth do think you’ll be poor? This is way more than many families have.
I agree that he should be doing more than 50% of the work around the house now he’s part time. But you won’t be poor.
When you retire you’ll have 2 state pensions- make sure you top up your NI contributions- plus any occupational pensions you have or can set up, no mortgage or rent to pay, and equity in your house that you could use to downsize. Financially you’ll be fine.

Op I think you mean for this to be about division of labour, that shouldn’t be fully on wage/ pension or outside work or else sahms and those of us in mw job would have to do everything in the house full stop

Why? When did she work full time so that he could have have a year or two off for training?

She worked part time for many, many years for 'lifestyle reasons'. Now he wants to do the same she's pissed off and resentful, despite him having brought far more to the table financially than her.

When it comes down to it, her resentment comes from deep rooted sexism, where she thinks that, as a woman, she's entitled to be financially supported by a man and not take equal financial responsibility for the family she jointly created.

Tell him it's his turn to support your career and work full time for a year or two while you study/retrain then up your own earnings.

Yeah, he knows

I see you. 💐

Ignore anyone whose main motivation for posting is to sneer at your situation or kick you while you're down.

Use your anger to propel you forward. Don't spend any more energy on your DH than you absolutely have to. Detach emotionally from the relationship and concentrate on building a life that fulfils you and a career that serves you better financially. You know already that he's not going to change. You've still got time to reclaim your life.

Courage! You can do it.

Your resentment is twofold. First, that he doesn’t pull his weight around the house, and second that you had and expected a fairly traditional set up where you did most of the domestic work and in return he would provide, but now that’s not happening.

You live with three adults, so you don’t need to be doing any cooking, cleaning, admin or clothes washing for anyone other than yourself. If they don’t do those things for themselves, tough. If they don’t do their share
of shared chores, they all need to be read the riot act.

But that was a choice to work part time.,and that choice always meant you would have less money

You earn together around 50 000 - this is more the national average. Pensions. Whatever comes, comes ....

Whats stopping you progress now? You could do 5 days, you could have dont 5 days for about 7 years. 4 days at £28k 5 days would be £35k.

Hes worked more, earnt more and paid off the mortgage. I think youd be very ungrateful to resent him, youre going to be a lot more secure becuase he paid off the mortgage

Like others, I sense from you a lack of personal responsibility for your own choices and where you are. Sounds like you would have liked to be financially supported by him and this is what you expected. Whilst I can see it’s disappointing it didn’t turn out how you expected, I think that most people could earn more than £28K at your age if they were determined and driven.

How old were the dc when you returned to
working full time?

The money you were paying on your mortgage can that not be paid into a pension or at least an ISA for the next 15 yrs until retirement

Sounds more like he sold his share in the business to the ither two partners for £50,000. If he didn't want to do that and they were all equal partners, then why did he sell?.