... and, in any event, I would not have thought that '20 x 60W lamps, in chandeliers' was something one came across in any 'normal house' even back then, was it?
Probably not, but when I count up the fittings fed off the lighting circuits in my parents 4 bedroom house I get
3 fittings (two single bulb, one circular flourescent) in the downstairs extension.
1 flourescent strip in the kitchen
2 fittings with 3 bulbs each in the lounge"
1 circular flourescent fitting in the dining room
1 fitting in the downstairs toilet, it's currently a LED fitting, but it was until-recently a flourescent strip so lets count it as that.
1 single bulb fitting in the hall
1 single bulb fitting each in three of the bedrooms
1 flourescent strip in the fourth bedroom
2 single bulb fittings on the landing
1 single bulb fitting in each of the two bathrooms
So that is 5 fluorescent strip/circular fittings, lets say 60W each for those for a total of 300W.
2 three-bulb fittings, if they were fitted with 60W bulbs that would be a total of 360W if they were fitted with 40W bulbs then 240W
10 single bulb fittings, if they were fitted with 100W bulbs that would be a total of 1000W, if they were fitted with 60W bulbs than 600W
So with incandescents in all the fittings that took traditional bulbs, that would be 1140W to 1660W depending on what bulbs were fitted at any given time, granted probably closer to the lower than the higher end of that range (and I suspect not all the single bulb fittings were rated for 100W bulbs). And I wouldn't regard my parents as especially posh, only a handful of rooms had more than one fitting.
So in the pre CFL/LED era I would say a single 5/6A circuit would be pushing it for a 4 bedroom house two was probably ample.
You can in theory go higher than 6A, but even with the SES/SBC rule removed, there are still potential difficulties. I checked two reputable brands of ceiling rose and both only listed a current rating of 6A. Cables run through lofts may also be subject to serious derating due to thermal insulation. There is also the question of manufacturers instructions on light fittings.
It does leave the question of what we should do when designing circuits now though, should we still allow the traditional 100W per point or should we allow lower based on the efficiency of modern lights. If so how much lower?