It has worked for many years and I would not be too worried about sitting astride a copper or steel boiler that a UK inspector has passed. I said they tend to be people with experience in making and running model boilers and that comes before they become an inspector. Many will also have been or still are involved in engineering as their day job. So will likely have experience before they even become a “new” inspector.
As I said the UK code does not have much in the way of technical guidance, it says suitable material but as far as I am aware does not state what is suitable. Again knowing what is generally accepted comes with experience or asking if you don’t have that experience hence my comment that the boiler inspector should be consulted early on and they will guide you through what they require. Can’t say where I picked it up from but must have taken it in some time in the last 43years which is how long ago I made my first steam engine and boiler