It's occasionally announced, but far more often it's not. I've heard one announcement about it ever, and I always do listen to the announcements if I am on VT. It is clear, however, that for a long time it has been the modus operandi of some train managers to tell everyone who asks to upgrade that it is £50 (or £40 from Liverpool) and to just make themselves comfortable in First and it will be collected later. I have never heard anybody who asked politely be told no or quoted the excess to 1st off peak single though, including a few people I know. Some of their train managers understand that the business is not infallible and faces many threats, as well as stalling growth, and try to increase revenue as much as practical as part of their normal duties and high level of customer service. This attitude is not always well promoted in the railway industry though so there seems to be a measure of success here.
I recall once asking for an upgrade to First Class on GWR from Worcester to Oxford, as there were 0 passengers in First Class leaving Worcester around midday! The guard quoted £30 or so which was the FDS rate, which I obviously did not take (I even already had a valid standard ticket...). CrossCountry have a policy of refusing to accept cash from Standard Advance ticket holders who change their mind on the train and want to upgrade to weekend first. Fortunately for them, the majority of their staff seem to ignore this policy!
