You (the account holder) do. Direct debits are instantly reversible for a month (maybe 2?).
You'll have to explain to the company that deducted the money why you reversed the charge though, and if it was legit after all you still need to pay them.
There is. It‘s 8 weeks (for any reason including "I‘d like my money back and rather do something else with it") or 13 months (for unauthorized charges).
"Immediate refunds. You can get a full and immediate refund from your bank (also known as an “indemnity claim”) for any payment taken in error."
https://gocardless.com/guides/posts/dd-guarantee-in-plain-en...