Welcome to General Aviation!
I think I could explain this far better with the aid of a whiteboard and a set of coloured markers! However, let's just start with variability...
- Aircraft in a GA airfield circuit may well have downwind speeds from 50-110 knots
- 3 degrees is only an instrument approach. What is normally appropriate depends upon the aircraft, varying from around 5 degrees for something like a PA28, to over 20 degrees for a really slow and draggy microlight like a TST or an XL. The XL and the PA28 may be in the same circuit.
- Circuit height varies also - from 600ft at some microlight airfields, to 1200ft at some places with a lot of irritable neighbours. 800ft or 1000ft is most normal. Some airfields change the circuit height at night.
- Most airfields reference QFE in the circuit, a few reference QNH
- Usually there's a single circuit, sometimes two (higher/faster, lower/slower): for example at Booker or Popham. They are defined geographically (rather than the time defined military circuit).
- As Whopity rightly says, at-least half of all UK GA airfields have a circuit modified for noise abatement.
- Different aircraft types, with different circuit heights, and often different circuit shapes and sizes will inevitably require differences in where you select flap, approach speed, approach power.... And if there are multiple aircraft behind you, select all of them as late as possible to help them keep their spacing. Alternately if you are in a microlight that accelerates for finals, select them as early as possible!
So basically you've correctly put your finger on the fact that there isn't a standard circuit in GA. There is a correct circuit at a given field, modified for your type, modified further to fit in with everybody else.
Strangely, it all makes good sense after a while. I think that some quality time with an FI and a whiteboard will certainly help - on the other hand, some quality time in the right hand seat with some experienced PPLs doing burger runs might help you out in seeing the variations in circuits (and much else, not all of which is quite so healthy
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G