Wow, this woman really mishandled the situation. (I would say "this couple" but the woman is the one gossiping about OP to third parties.) First of all, if she and her husband got food poisoning from OP's food, she should have let OP know, right away, so OP could track down the source. By doing nothing, but judging OP anyway, and by sharing that judgement with others, the woman has cast a pall over OP's social standing in their friend group - others may now be afraid to eat OP's cooking, but likely wouldn't say anything to OP so she could defend herself (or fix the cooking problem, if there is one, which I doubt). It's almost like defamation, but subtle, and low-key, and hidden where you can't see the poison.
Second of all, the whole explanation that it's because OP uses a different grocery, makes zero sense. First of all, if there is something like salmonella in chicken, that usually doesn't show up as a stomach illness for weeks. It takes months for recalls to happen as groceries track down the source of the problem. Second, as OP pointed out, everyone else who ate the same food was fine. The idea that some other ingredient (and the friend doesn't give any indication as to what that ingredient might be) is below normal standards, is highly unlikely in a wealthy and Westernized country like the UK, where there are strict oversights on food quality. In other words, the woman's claim that OP's ingredients were problematic because she shops at a different (i.e. less fancy) grocery store than the friend, simply doesn't hold water and just comes off as pure snobbery. She's basically saying, you are too low class for me to eat your low-class food.
Third, once the woman realized her rudeness had been exposed, instead of apologizing, she doubles down! Common sense and the slightest bit of etiquette should tell her that she should apologize profusely for gossiping about her friend behind her back.
The overall problem is that this woman's behavior shows a total lack of respect for OP and a total lack of interest in maintaining a friendship. She's essentially saying that the only way the friendship can continue is if OP tacitly agrees that her food is inedible - they can eat together, but only food that OP hasn't made!!! It's condescending and rude. And yet somehow the OP's husband expects OP to continue to have this boorish woman over to her house and host her again? Woooooooowwwwww!!!!