Very early in the regulations we have:-
314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced
by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.
If you look at (iii) above that is the basic rules to say don't fit a socket to a lighting circuit. Reason is simply to stop some thing being plugged into the socket from tripping out the lights.
It was years ago illegal to use power products from the lighting circuit. There were two meters one for lights and one for power and lighting cost less than power. Not sure of dates but likely pre-first world war.
But by the time of the second world war all on same meter and my granddads house had two 15A sockets so taking power with adaptors from the lights was wide spread but by end on second world war more sockets were being fitted my dads house had 5 sockets. However the lights also had no earths so because of lack of earth talking power from lights was very dangerous.
By the 1960's they changed and fitted earths to lights but there are still many houses without earths and also RCD's were not fitted to lighting circuits until 2008 when the bathroom lights had to have an RCD.
So in general the only time when a socket is fitted to the lighting circuit is to supply TV boosters fitted in the loft and even this really breaks the rules. The only other non lighting use for lighting supply is the fan in bathroom and fire alarms. In all cases a very low current consumption.
The regulations say the protective device (fuse) should be fitted at "point of reduction of cross-sectional area" but it allows 3 meters (434.2.1) if certain rules are obeyed before the fitting of the protective device. The socket single or double or the fused connection unit is considered as the protective device since the maximum fuse size fitted is 26A and twin and earth clipped direct is rated at 27A. Clearly where ref method 100 is used then only rated at 21A but a risk assessment shows it is unlikely in real terms to exceed the 21A for enough time to allow the cable to over heat. In a plug the 13A fuse produces more heat than the modified plug since the finger protectors were added can radiate over a prolonged time.
"ban-all-sheds" 20A limit for a double sockets reflects this heat dissipation problem two plugs so close heat each other up. All fuses produce heat this is how they work. So for an immersion heater which runs for an extended time we will not use a fused plug but use a dedicated supply. We may use a unfused 15A plug so it can be unplugged for easy maintenance these should produce no heat.