Using a single bank account as the backend of gfrbank.com

We've been thinking about the possibility of creating a single bank account for GfrBank and then building gfrbank.com on top of that.
We'd ideally choose a bank account with online access that doesn't require a physical device to sign in - as we think this would increase the chance of us being able to automate some of the interaction we'd inevitably have with this backend bank account.
We'd have to understand some of the restrictions on the types of bank account we can sign up for. Maybe we could use some kind of society account.
We'd be looking to regularly import transactions from the backend bank to gfrbank.com to provide a read-only view on GfrBank customers' accounts/transactions.

Transferring money in

GfrBank customers could electronically transfer money in to the backend bank by using the backend bank's sort code, account number and a reference that we give them.
NOTE. This is how I pay in to the London Capital Credit Union - I pay in to their single Co-op account, supplying my member number as the reference.

GfrBank customers could initiate a transfer out of their account from gfrbank.com.
We'd ensure they had enough money in their account and then instruct the backend bank to make the transfer.
We'd ideally instruct the backend bank programmatically but it's possible we'll have to do it manually.

This could all happen within gfrbank.com as the backend bank doesn't need to know about these transfers.

GfrBank customers could initiate a payment from gfrbank.com by supplying the sort code, account number and reference of the target account.
This would work in the same way transferring money out of their GfrBank account.
It's possible this might cause confusion for the recipient, as the origin account will be something like "GfrBank", rather than, "Chris Roos". Hopefully the transaction reference will go some way to avoiding this becoming a real problem.

We think we could offer a version of a Standing Order.
GfrBank customers could set-up regular payments in gfrbank.com.
GfrBank would instruct the backend bank to make the payments on the dates specified. This would work the same as "Transferring money out".

We don't think we can offer Direct Debit facilities.
The backend bank account wouldn't know anything about the GfrBank customer's balance which would mean a payment could result in that customer going overdrawn with GfrBank (even though our backend bank account might continue with a positive balance).
Even if we did have Direct Debits set-up in the backend bank account we're not entirely sure that it'd be possible to associate a DD payment with the GfrBank customer that requested it.

We don't think we'd be able to offer debit cards as they'd only be available to the named account holders of the backend bank.

If we had a society account, could members of the society (i.e. GfrBank members) have debit cards?