Wire
Trevor, at least in the U.S., wire is sized by guage, and for a given wire guage, the number of strands does not affect the current carrying capacity ("ampacity" in the electrical trade). Is wire sold by mm diameter in the U.K.?
Yes, it is a bit strange that your wire with the larger number of .2mm. strands has a higher ampacity, but the difference could reflect the intended application and temperature rating of the insulation. In particular, if what you are describing as general purpose hookup wire is intended for non-vehicular uses, it may have insulation that is rated at very low temperature. Some ordinary "hookup wire" has amazingly low temperature tolerance and will melt back when you solder the wire. Such wire would be totally unsuited to vehicular applications.
Speaking of solder, in general, PROPERLY crimped, high-quality crimped connectors are more reliable than soldered connections for vehicular use with vibration exposure. A soldered joint can provide such rigid support for the wire that it will be more subject to fatigue failure right at the end of the solder joint if it is subjected to continuous vibration. That is also a good reason to use cable ties liberally to minimize flexing of the wire joints.
In addition to temperature rating, insulation on wire intended for vehicular applications should be tolerant of sunlight (depending upon the styling of your bike) and oil and gas exposure. For motorcycle wiring you would also want abrasion and fatigue resistance, especially if you ride a Harley. (Sorry, as a BMW K75 and R65 rider, I couldn't resist that.)
Considering the implications of an electrical problem in your bike on the road and the relatively small amount of wire in even a complex, modern motorcycle, I think you should get the highest grade wire available and be sure it is intended for vehicular wiring. Pay the extra price and consider it a road insurance premium.
Have fun.
awright