To have changed my mind about letting some people stay in our house while we are away

One of the school mums posted on a WhatsApp chat asking if anyone could help put her in laws up while they visited the UK from South Africa. It would be for a week.

Get a dictionary. Find the 'H' section. Then find the word hotel. Show it to her.

a building where you pay to have a room to sleep in, and where you can sometimes eat meals

It's obvious why she's done it but she's still a cf. And for someone who can be that cheeky I'd worry about how they might treat the house.

The school mum is a piss taker for asking. Why in god's name can't they sleep in a hotel or air BnB? Or sleep on an airbed in this woman's lounge? They want a whole house, for a week for free. To house people you've never met or even heard of? Definitely make your excuses.
Even if it puts her out, tough. She shouldn't be imposing her in-laws on near strangers.

Just say you are very sorry to have messed her around, but you won't be able to offer the house for that week after all.

No need to explain, but there is a need to let her know straight away so she can spend the time looking for alternative accommodation.

Contact her now before she turns up and say circumstances have changed and this no longer works for you.
But you need to act quickly. They'll just have to find an Airbnb

It’s crazy that someone would ask that in the first place. And I’d never agree to complete and utter strangers staying in my house. But I guess it depends how long there is until they’re supposed to be arriving as to how unreasonable it would be to back out now. If they’ve plenty of time to find an affordable alternative then fine. If they’re arriving tomorrow and all reasonable accommodation options are fully booked it would be a bit shit.

If you are going to back out you need to do it now!!

Why the sudden turnaround though?

Never in a million years would I have agreed in the first place.

Why on earth would you offer your house up to complete strangers?? Just say no it no longer works for you.

Are you a compulsive people pleaser OP? Did you jump in to offer? I have got myself into these situations before. Is she doing some kind of house inspection to make sure its suitable? Drop out as quickly as you can. Did DH/DP agree to this as well? You just need to wriggle out of it as quickly as possible to give her as much time as possible to sort something else out. It's a massive favour when AirBnB and hotels exist - you'd be saving someone £500+ so I do wonder why she can't sort them out herself - but you really really don't need to do it.

When are they arriving?
six months time : fine to back out
tomorrow : suck it up

Unless they're literally arriving tomorrow I think it's totally reasonable to say on reflection you don't actually want people you don't know staying in your house while you're not there.

It's a massive ask from her, why can't they book an air BnB like most would in this situation?

Just say that your family circumstances or situation have changed so they can no longer stay.

You offered your home to a school mum for her family memember???? Absolutely no fecking way ….only my children or my mum could stay at mine when I’m away…..not even other family …just say no

When are they meant to arrive?

if the arrangement today is to allow her to say yes or no to your house, it would seem fair that you could still cancel from your end. If she’s coming to pick up the keys because they are arriving tomorrow you’d be giving her a complete nightmare