I don't think fitting an RCBO to an existing circuit is notifiable. You just remove the existing breaker and replace with an appropriate rating RCBO (preferably from the manufacturer of the consumer unit), they cost £10-£15.
Notifiable stuff is generally the addition of a new circuit which includes a new CU, some outdoor stuff etc. etc.
I don't think fitting an RCBO to an existing circuit is notifiable. You just remove the existing breaker and replace with an appropriate rating RCBO (preferably from the manufacturer of the consumer unit), they cost £10-£15.
Notifiable stuff is generally the addition of a new circuit which includes a new CU, some outdoor stuff etc. etc.