You have to inform the bank and the direct debitee (another new word?) but as long as you do that it's sound.
You don't need their permission but you have to do it outside of their 5 working day clearance period which is fair enough
Yes, A direct debit is exactly what it says, an authorisation for the The company/organisation to take money from your account . They can take any amount at any time. You can recall the DD if you see it in time and go to the bank.
If you want to set up a non changeable payment method then set up a standing order. A standing order is for a specific amount at a specific date for a specific time it can not be changed by the payee
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grasscarp ALL DDs are variable that is why you should only use a standing order. A DD does not specify an amount it just allows the payee to take whatever they want from your account at any time.