Struggling to support my 91 yr old mother

That's a really good idea Avalon.
Then mother can enjoy a bit of shopping (which as others have said, is more a treat than going for a coffee to her) but you may still have time to go to a garden centre too.
And just checking - is your mother relaxed about eating in public? As my MiL had mild swallowing issues & ill-fitting dentures which the dentist couldn't really solve without implants, & I didn't realise for a bit why she didn't want to go out to eat/ drink, but this was why.

Instore not Istore

Unfortunately Sainsbury’s has closed all its I store cafes or you could have combined both. I think you have to tell mother it’s either Sainsbury’s or a trip out. If she chooses the former perhaps you could buy a cake I store and take it home to share together with a drink. Having said that 3 hours does seem a long time for her to be walking round Sainsbury’s with nowhere to sit down.

Could you do the shopping for basics online which you can then just put in as a repeat order and take her in store to shop for the rest?

I agree that you need to be firm. You might phrase it as an "I" message and validate her feelings: "Mum, I realize that you like to do the weekly shop and that it's a fun outing for you. However, I feel that it's getting to be too much for me to take you to do that. This is what I'm willing to do as an alternative."

Remember, if she throws a fit, you're not obliged to provide her with an audience. "Mum, I recognize you're upset. However, this is what I'm able to offer. If you keep shouting, this conversation is over." Then follow through with it.

You're not obliged to martyr yourself for her. She can accept an alternative option, or she can see if there is someone else who is willing to take her for a 3-hour shop. Those are her choices.

I think the key is to tell her you want to take her out for a weekly trip but can't do that and take her shopping as well.

You don't mention whether you can do online shopping, or whether she can be taught how to do it on a tablet maybe. It would give her a great sense of achievement if she could do this. It's really not difficult once her favourites are saved and her banking method is set up at the start. It just requires adding the items to the basket and pressing pay now. She would feel in control and would probably enjoy it.

I hope this would be a good solution, but if she or you can't manage online shopping then you'll need to be firm and say you can't take her. Do not give in no matter how many tantrums ensue. I think you are doing as much as you can which is very generous.

Could your treat of a National Trust coffee shop be a treat for you and her visit to Sainsburys be her treat for herself buying her choice of food and seeing all sorts of people? At 80 I do love choosing my own vegetables but I also love a National trust coffee and cake. Take her to a smaller Sainsburys as 3 hours is a killer shop but my friend who is 90 loves going around a shop but never seems to buy anything which is her choice so I think she just likes seeing the world out side her own four walls.