Is it true that one can buy such a pass and take possession of it immediately at one of the station ticket offices? No delay?
You may want to private message Nigel (he responds to many questions on this board). He works for one of the UK train companies.
I'll post in the open a similar answer to that I gave privately. The answer is yes, if the booking office is open (manned) and if it is not a peak hour. Most offices, particularly smaller ones, will only deal with tickets for immediate travel at certain hours, like 0600-0930, so that transactions like railcards don't hold up the queue of people just wanting simple tickets and then jumping on the train. Proof of age and identity is required, passport. So, generally, yes.
To avoid confusion for people who might be unfamiliar with this offer, I'll point out that we're talking about a senior railcard. There is no senior railpass. Here's a quote from the National Rail website: "If you're 60 or over our Senior Railcard gives you 1/3 off most Standard and First Class rail fares throughout Great Britain." For more information, see this page.
Good catch, Tim. My sleepy eyes saw senior and missed the "pass". Yup, what I said is about the Senior Railcard. I know from nothing about passes.