Many different voltages were in use in different areas, from 100 to 250, even different frequencies (and DC) before the national grid standardised all new supplies from around 1931 onwards. The highest voltage in normal use was 250V, so most single-phase accessories were rated for that voltage, and most 3-phase equipment for 500V.
The majority of supplies were 200V or higher, therefore 5A was usually sufficient for 1kW and the 5A multiples approximately corresponded to the kW rating of typical loads.
Typical domestic applications were:
5A - lighting, small outlets
10A - heating, fell out of favour in the 1930s
15A - heating, water heating (stored), large domestic outlets
20A - radial circuit for small numbers of BS1363 outlets and FCU-attached appliances (after 1947)
30A - cooking, ring final circuits for BS1363 outlets (after 1947), main fuses for small installations
45A - large cooking, submains, showers (not then popular)
60A - main fuses
The UK was quite old-fashioned in terms of OCPD choice, we continued to favour rewireable fuses when Europe was increasingly using HRC cartridges. Fuse wire was sold in the above ratings and these were carried over to HRC fuses when we started to get into those. You can still buy cards of fuse wire in hardware stores, with the most popular domestic ratings, 5, 15 & 30A and sometimes 20A.
As the popularity of MCBs increased the first really successful product series also used the multiples of 5A e.g. Crabtree C50 (that's a model number, not a rating), Wylex plug-in and NB types, MK LN / Sentry and other early BS3871 models. We migrated to the Renard number ratings with the rest of Europe in the 1980s. Series that were current at the time existed in both flavours. E.g. MK Sentry Mk.1 can be found with 5A, 6A, 10A, 15A, 16A etc ratings. Many products such as industrial switchgear also made the shift (30-32A, 60-63A etc) with the same basic products being rebadged with the new rating, perhaps with a styling facelift.
