the rules vary slightly from borough to borough (or at least they do in London)
observation periods depend on the Borough also
every council will have a Traffic Order setting out the Waiting & Loading articles which apply to that borough / council.
In London at present we have widespread confusion - the rules have been changed by some & not by others
Some London Councils have an unlimited before 11AM & 20 Mins after rule which applies to SYL DYL & variously to P&D & Meters & Residents Bays
Some (London) have applied a 40 mins at any time rule
there are numerous key loading adjudication cases within the information section of the website.