This may be well out of date, but at the time you are researching, Catholic couples had to marry in CofE churches for it to be legal, as RC priests weren't (and still aren't... as far as I know... unless it's changed in the last 10 years or so) able to perform a legal marriage. Only when registrars were introduced could a legal marriage take place in a catholic church, when a registrar could attend to issue the certificate (hence the "register office or registrar attended" quote you find on county BMD entries). Registrars could (and can) issue the certificate in advance of the ceremony, providing it could/can be kept in a safe.
I have RC ancestors who baptised all their children from 1798 in St Mary's RC, Highfield St, Liverpool... but married at Walton St Mary in 1798, their local CofE church.
I'm also looking for pre 1801 RC records from Little Crosby.