Ratesetter does not allow sort code/account number changes on their website, but it’s doable via email or phone call. To be fair, it needs to be changed first, before I can even try to setup direct debit. I sent them an email to ask my bank from Metrobank to Monzo and this was their response:
Unfortunately we would not be able to accept this type of bank account for investment purposes. The reason is due to it being a purely mobile-only based bank which makes it difficult to verify the actual bank account.
I pointed out BACS rules, gave them link to Zopa’s tweet, as well as link to this thread 
(“hello!” to any RateSetter employees if they went that far to read it! Now please approve Monzo sort code!).