BackPacker, quite why anyone who states that they are "too lazy to look it up in my books " thinks that someone else should spoon-feed them with the answer is beyond me....
However,
"If you flew a 1hr flight with a FI, who is supposedly current and everything, and that FI signs your logbook and the revalidation slip in your license, then why does the CFI from the club be involved at all?"
The FI may conduct the training flight, however, only an authorised Flight Examiner (in the UK at any rate) may sign the Certificate of Revalidation in the licence. There is no requirement for any Club CFI or HoT to become involved.
"Or is this an additional club rule, that you need to be checked out on an aircraft by a club FI before you're allowed to fly club aircraft (possibly for insurance reasons), and that this club checkout happens to coincide with the official proficiency check?"
Again, nothing mandated, but that's a reasonable way of killing two birds with one stone.
"I'm going to have to do my proficiency check for the first time, in a few months or so, so I'm curious how this works, or how it worked in your case and what the issues really were."
Really very straightforward. Take-off, 15 minutes of VFR navigation, a couple of steep turns, a couple of stalls and an off-aerodrome practice forced landing. Then back home for a normal circuit, lo go-around, EFATO, flapless circuit and simulated aborted take-off.
The 1 hour training flight may be flown within the 12 months before expiry of the rating; the SEP revalidation LPC may be flown at any time. But if flown within the last 3 months of validity, the Rating validity will roll forwards 24 months from the date of expiry rather than from the date of the check.