I could quite quickly produce you a 'certified' copy of a passport. The ECO would have two options to check if it's a forgery - either phone up the certifier or just access a copy of the passport from their system using the passport number. It's 100% certain that they are going to check the sponsor's passport/immigration records anyway so the certification of the photocopy is actually meaningless.
And the fact that people on here haven't bothered to certify their copies proves it isn't mandatory.
It's slightly different if you have a non-UK passport...