Why does Apple need my account number and sort code for payments?

Since you received that from an email, the standard rule still applies.

Never provide personal or financial information from an email link.

I am still not clear what action was taken on your part to bring this prompt up as it appears to just come from your Gmail and don't know the connection with the monthly payments. Apple does make this Support Article about communication it may provide.

Identify legitimate emails from the App Store or iTunes Store - Apple Support

Information about your purchases on apple.com are available here:

Your Account - Shopping Help - Apple

And as always, you can contact Apple directly:

Contact Apple for support and service - Apple Support

Don't know what monthly payment plan you are setting up or where you are attempting to do it. Please add a screenshot if you are able to without providing any personal information.

You want to make sure it is not a phishing attempt.

Have you even actually purchased anything from Apple?