You can do what you like. You could inspect a ladder and say "this meets BS whatever:whenever" if you want. You could note that it doesn't meet current version but does meet previous version, or indeed that it doesn't meet any version. Since satisfying a particular revision of a BS doesn't normally mean much (it doesn't absolve you of the need to RA or provide suitable or sufficient etc) I'm not sure I understand the significance of the question. Like the recent mis-marketing of the first aid kits BS by suppliers - the BS defines a standard for what should be in a standard kit, but there's no law that says the kit in a workplace must meet the BS, and providing a BS kit doesn't get you out of assessing your requirements and providing an appropriate kit. In the few places where BSs are mandated in statute, it always (as far as I know) refers to a particular revision, since otherwise we would be in the situation that a BS drafting committee would be changing the law. Whether a statute that refers to a BS applies retrospectively depends on the statute, but few do (I think).

Of course British Standards have relevance. They are relevant to whether you say a thing meets British Standards or not. In some cases you will want to say that the thing meets the standard, eg if it is your policy that these things should meet that standard, or you may want a means of showing that the thing is an acceptable thing without drawing up and justifying your own specification from scratch. In other words they have the relevance you choose to give them.