At minimum, if you choose to make your account information available to a third party, they will be able to see:
The account name and type
The available balance
Details about transactions you have sent and received
If you consent to detailed access, they will also be able to see the account number and sort code.
If you give a third party permission to make payments on your behalf, they will be able to see:
The payment amount
Whether it was inbound or outbound
The transaction ID and reference
The payment status (completed, failed, etc)
Booking date and time
Value date and time
If you consent to detailed access, they will also be able to see your account balance and information about payees.