Replacement boilers are controlled by the Building Regulations. If the person doing the installation work is: "A person, or an employee of a person, who is a member of a class of persons approved in accordance with regulation 3 of the Gas Safety (Installation and Use) Regulations 1988. Corgi, etc." then they are exempt from the requirement to submit an application to building control. You are advised to ask for proof that the installer is adequately qualified. This exemption only applies where the boiler is not part of general building work being done at the same time.