Great detective work, guys! 

And all done by us using the information on the government site; which is free for all. That information states that a Draft Order that is laid in Parliament and approved, is "made" into a Statutory Instrument when it is signed into law; usually signed by the relevant Minister.
"Draft affirmative SIs laid in Parliament need to be approved by Parliament before they can be made (signed into law) and brought into effect as law."https://www.parliament.uk/about/how/laws/secondary-legislation/As ksand just found, the order wasn't made (signed into law) until 18 December and the legislation for this Draft Order (link above) said it would start 21 days after the Order was made into a Statutory Instrument. The Draft made into law on 18th December 2018 means it starts on 8 January 2019.
It didn't make sense how it could start on the 18th December as some professional law sites had claimed because; as mentioned above on 9 December; it was still showing as a Draft Order (not made) on the government's legislation site, even though the Draft had been approved by Parliament at the end of November. The government merely made this Draft a Statutory Instrument (signed into law) 21 days before they wanted the rise to come into affect.