It would appear that which regs you have to comply with pertain to the age of the vehicle you are driving/riding at the time (of being caught), because there is no requirement to have to upgrade old vehicle speedos to meet latest regs (yet!).
Since being part of the EEC we now have to comply with EU regs which in turn have latterly been harmonised with UN regs for vehicles.
The last ammendment to EU stuff was 2004 and is (mainly at section 5.3:
[url=http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:42004X0331(01)&qid=1517485917963&from=en]EU 2004 regs you can see that this mentions +4KPH
And just to be more confusing, since year 2000, 2 and 3 wheeled vehicles have their own set of speedo regs:
None of which is harmonised with UN regs!!!
There are previous ammendments in 1971, 1980, 1986 to UK and EU C&U regs.
The vast amjority of these regs cover electronic speeds, mechanical speedos are subject to different C&U regs (and generally were not updated as mech speedo tech did not advance that much), for instance most 1950/60s speedos were subject to 1939 speedo C&U regs.
All this C&U legislation old and new is exactly why plod nowadays does not stand a chance of being 100% correct, they would have to be a walking reference book.