A bank can’t take money from your account without your permission using right of offset unless the following conditions are all met:
The current account and the debt are both in your name. The position is a bit more complicated with joint debts and joint accounts
The current account and debt are both with the same lender. A bank can’t take money from your account for a debt with a different company
The debt they’re taking money for is in arrears. They can’t take money by right of set-off if the debt repayments are up to date
They’ve warned you clearly in advance that they may use right of set-off if you don’t contact them or pay back your arrears
They’ve taken your individual circumstances into account and whether taking the money might cause you hardship
In practice it’s very rare for banks to exercise this right. However, if your bank has used right of set-off they must contact you to explain how you can avoid it happening again in future.