//I thought the 20 mph was applicable only in school zones.//
In London there are boroughs where almost the entire area is 20mph.
//If they do follow the guidelines you will be allowed to drive at the posted limit +10% + 2mph which means if you don't exceed 24 mph you will be safe from prosecution.//
First of all that is incorrect. Enforcement BEGINS at (Limit + 10% +2mph), so 24mph in a 20mph. So if you don't exceed 23mph you will normally be safe from prosecution, 24mph will see enforcement.
The guidelines referred to are those of the National Police Chiefs' Council (NPCC). The ACPO (Association of Chief Police Officers) was abolished in 2015. The NPCC's guidelines are here:
https://www.npcc.police.uk/2018%20FOI/Operations/030%2018%20Speed%20Enforcement%20Guidance.pdf
The table of enforcement thresholds is at para 9.6. However, you should note paragraph 9.7:
"These guidelines do not and cannot replace police officer’s discretion. Where an officer decides to issue a summons or fixed penalty notice in respect of offences committed at speeds lower than those set out in the table, he or she must consider the tolerances of the equipment used to corroborate their opinion. Police speed equipment are tested to work with a maximum tolerance of +/- 2mph up to 66mph and 3% for all speeds above 66mph, so it is possible to use these tolerances as a prosecution threshold. Moreover, in particular circumstances, driving at speeds lower than the legal limit may result in prosecution for other offences, for example dangerous driving or driving without due care and attention when the speed is inappropriate and inherently unsafe."
You'd be better off setting any such limiter at 20mph.