I had a new this year loco from a well-known and respected builder on my bench until a few days ago. The boiler needed a repair, testing and re-certification for the owner's peace of mind - nothing more. It had no identifying mark.
My question to you is then: Who wants to see the certificate? And why?
You make it sound as though a clandestine police force is going to break in to your home and take your engines away. There is nobody policing this, other than event/ME organisers who can, frankly, ask you to do anything they like, including saying that any existing certificate is invalid and only their own tester can issue one. At which point I would wish them well and walk away.
Bear in mind too that WHO issues the certificate and WHAT equipment they use also has a bearing. A good friend who has been building boilers for 30 years tests and certifies his own boilers, but because he is not a 16mm Association tester using Association equipment his certificate does not meet the needs of the Association's insurance company. Instead, they would prefer a certificate from somebody who may or may not have any experience in testing boilers, provided somebody has said they are OK to do the testing and they have an Association rig.
I feel that mountains out of mole hills are being built here on this forum.