Morning Beth - a quick question but not an overly simple answer I'm afraid.
The official guidance we refer people to is held on
GOV.UK and is relied upon across government re how to certify a copy
From a purely registration perspective a certified copy is one where a conveyancer has certified that it is a true copy of the original and endorsed with their name and address
Our requirements may vary depending on the type of document/application involved but we can often accept certification done by a named individual or in the name of the firm - Rule 217 refers to the name and address aspect re a conveyancer but also allows us discretion so certified in a firm's name can be accepted if appropriate
I would agree with you re best practice but am also aware that some firms will not view it that way and will sign/certify in the name of the firm