I think for FRA members as long as you sort entry in first couple of hours you're generally ok, even for something as obviously popular as Fairfield... Long Mynd Valleys entries were available for several days at least... Might be more difficult for non-members.

The only fell races i've noticed where places really do go in minutes are Cader Idris and Snowdon (i can't speak for Ben Nevis as never attempted to enter).

I see that Lower Borrowdale Skyline, a champs race last year, has been postponed/cancelled this year as they only got something like 37 pre-entries.