As a guard, I will let any member of the railway family on my train and send them to 1st Class. Unfortunately you will get the odd mardy guard who will go by the book, but when I travel on other tocs I make a point of finding and asking the guard ASAP and they have always been as good as gold.
Absolutely. The asking about buying is a courtesy really. Most will say not to worry, as will I. If I'm allowed free pass, I always offer to help if needed.
I do make a point of having a priv ticket, on other TOCs, if I'm having a few beers tho. I feel it makes a distinction that I'm off duty and in no way capable of helping!
Likewise, if you flash your police ID, I've no issues at all with you travelling free, but there's the unspoken knowledge that, if things do go wrong or I need you, you'll step up and help- indeed, I asked a plainclothes to help a lady and her shopping through the station when my service was replatformed at the last minute.
Technically, yes, it's not allowed and should be declared etc etc, but there's no way I'm gonna be the first guard to make a stand over it!