Well, this is all rather premature given that you're not married yet, although I suppose it's sensible to think about how you will manage childcare before marrying and TTC.
My main concern here is that your DH seems to be very heavily influenced by his family, and they are all putting a lot of pressure on you already, which doesn't bode well for the future. It's up to you and your DH how you decide to raise your child. Your DH needs to make his own mind up, and he also needs to respect your views. If and when you marry and have children, you have to prioritise your marriage and family unit over families of origin. This might cause tensions if your DH's family are opinionated and he is easily influenced by them.
I also find it sexist and hypocritical that he/they are all pressuring you to give up or reduce your working hours, without even considering the possibility that he could do so.
You talk about difference in salaries, but £22k v £21k is a very insignificant difference. You talk about opportunities, but that will become a self-fulfilling prophecy - if you (as a couple) prioritise his career, he will take the opportunities and increase his salary, while your opportunities are limited and your salary stagnates. If you want to continue to progress in your career - in terms of salary and opportunities - you have to protect it. That means thinking carefully about the impact of becoming a SAHM or going part time, and considering a compromise - for example, would 4 days a week still allow you to progress and protect your earning power? Don't forget pension contributions, too.
FWIW I think that in an ideal world, both parents would work 4 days a week, which wouldn't be too big a sacrifice career-wise. That only leaves 3 days a week for childcare. If there is a grandparent who is able and suitable, they could do 1 day a week and you could use a childminder or nursery for the other 2 days.
Btw childminder is generally thought to be better than nursery for children under 2/3, but of course it depends on the childminder and nursery. A good childminder is ideal but a good nursery is probably better than an average childminder. Anyway, you have PLENTY of time to make that decision!
Meanwhile... consider whether you and your partner will be on the same page when it comes to the big decisions, or whether you'll have to fight with his family as well as him.
When it comes to in-laws, the sh*t sometimes hits the fan after the first grandchildren are born. (It did for us.) Are you confident that your partner will back you up?