They are available in the sizes I mentioned, plus 2A – https://www.farnell.com/datasheets/1683350.pdf – and your comment made me actually look up the regulation out of curiosity as I know I have seen some of those other sizes in the lab! The wiring regs are enforced in statute by The Plugs and Sockets (Etc) Statutory Instrument 1994 [1] which mandates the compliance of two British standards, BS 1362 and BS 1363 for fuses and plugs respectively. The exact wording of BS 1363 (at least the version of it I can access) is
> [...] all rewirable plugs shall be marked on the engagement surface with the rated current. All non-rewirable plugs shall be marked with the rated current of the fuse link fitted, which shall not exceed the value given in Table 2 for the appropriate size of flexible cord
Table 2 itself prescribes a maximum fuse rating of 3 A or "(5 A)" [see below] for a conductor cross-sectional area of 0.5 mm^2, and 13 A for all larger conductor areas (0.75, 1, 1.25, and 1.5 mm^2). It is entitled "Rated current and maximum fuse rating in normal use, and load for flexing and cord grip tests related to size of flexible cord"
> [...] The figure in brackets indicates the fuse rating when a non rewirable plug is used with certain types of equipment where the use
of a 5 A fuse link is necessary because of the high instantaneous inrush current
So there we go, I think – we're all sort of right. Thanks for sending me down this rabbit hole!
> [...] all rewirable plugs shall be marked on the engagement surface with the rated current. All non-rewirable plugs shall be marked with the rated current of the fuse link fitted, which shall not exceed the value given in Table 2 for the appropriate size of flexible cord
Table 2 itself prescribes a maximum fuse rating of 3 A or "(5 A)" [see below] for a conductor cross-sectional area of 0.5 mm^2, and 13 A for all larger conductor areas (0.75, 1, 1.25, and 1.5 mm^2). It is entitled "Rated current and maximum fuse rating in normal use, and load for flexing and cord grip tests related to size of flexible cord"
> [...] The figure in brackets indicates the fuse rating when a non rewirable plug is used with certain types of equipment where the use of a 5 A fuse link is necessary because of the high instantaneous inrush current
So there we go, I think – we're all sort of right. Thanks for sending me down this rabbit hole!
[1] https://www.legislation.gov.uk/uksi/1994/1768/made