Okay, leaving aside the perversity of posting on a feminism board with the opener "I am not a feminist", here's a check list of stuff to make sure you have sewn up properly, financially and legally, before you become a SAHM (since you talk about your partner rather than your husband).
Joint accounts. If he takes the view that his money is now his because he's the earner, walk away from the relationship, do not have children with this man.
In the event of death - make sure he is well insured. Check the value of your house, because the one advantage of marriage which cannot be reproduced in any other way is that you are exempt from inheritance tax on your late spouse's estate. If you live in London/SE, chances are your house, even a very modest house, will take you well over the inheritance threshold. Make sure your insurance takes this into account. Likewise, insure yourself - if you stepped under a bus, he'd have to pay for a nanny, cleaner, etc, or radically downscale his job, earnings potential and lifestyle.
Possible separation - after all, something like 1 in 4 first marriages end in divorce, so although you think your relationship is for ever, the odds suggest that it's a possibility you should take seriously. Make sure the mortgage is in joint names and that you agree to split the proceeds of any sale 50-50 in the event of you separating. Note that it does not matter that after you become a SAHM the payments will come 100% out of his salary. You are in effect saving him the cost of a nanny, cleaner, cook-housekeeper. The assets of the relationship should be tied up legally to be 50-50. If he is not happy with this, again, walk away from the relationship now before you have children.
No doubt others will have more suggestions.