I think the OP has taken what everyone else said in good spirits and I'm sure he will remember for the future. I think we can all be prepared to cut him some slack.
As for signing in, I am sure some people will have their own take on things but the bottom line is you can be required to sign in as it is private property. I do not recommend kicking up a stink over what is a simple request made in good faith. The important part is to enjoy whatever trip you are on.
As such, I have locked the thread as all useful and sensible advice has been given. I don't see it necessary to get into all the technicalities of what others think should and should not happen re: signing in. It complicates matters unnecessarily for the OP. That discussion can be had in a separate thread if people feel strongly about it.
Welcome by the way.
If they must require a "trainspotter" to tell them that they are there, I would say that they should have a seperate "visitors" form.
They should do. If the OP has any concern over that one, he may wish to contact GWR about it.