British Standards and the like are not retrospective by themselves. The Fire Risk Assessment, which has to account for technical progress, has to determine if the existing precautions remain tolerable without modernisation - in many cases they are still suitable (especially if other aspects have been modernised.
Example - a hotel in the 1970's could comply with fire safety legislation with either no smoke detection or provision just to the stairs and corridors - this would no longer be tolerable as lack of any, or just limited, detection in any sleeping accommodation has been proven to be an unacceptable risk to life so the current standard requires detection throughout most areas of the hotel including bedrooms and it would be expected this is retrospectively applied & legally enforceable. On the other hand the same hotel would have original fire doors with a wooden stop on the frame instead of the current standard of intumescent strips and smoke brushes but could (& have in legal cases) justify not replacing them where they were in good working condition & the premises had more detection than when first approved.
They only need to work to the 2025 standard for service intervals and actions and for major additions/new systems - the system itself does not need to be current as long as it still performs adequately and the FRA says such.