Hello Delia! I hope you’re well. I can think of a couple of solutions that might work.
Talk to your bank - It might be worth telling them that this is an account for someone with a learning disability and you will be considering switching to another bank if they don’t offer this. I don’t see why they couldn’t theoretically do this but you may need to actually make it clear that you will be switching banks if they don’t, even if you don’t intend to. You could mention the Equality Act and this being an issue of accessibility and see if they change their tune.
Post Office card - A Post Office card allows someone to collect benefits, tax credits and state pensions, although you can’t pay any other money in. You can only take money out using the card and PIN at a Post Office, but it doesn’t allow for overdrawing, which might be an option.
Digital bank accounts - There are a couple of providers of digital bank accounts who may be flexible and understanding and it might be worth contacting them to see if they offer accounts that can’t be withdrawn. There are two ones, Monzo and First Direct, that might be flexible.
Accounts for teenagers/children - This might also give you more control. I’ve found goHenry which might be something that you can use.
Hopefully these solutions can help!