1st January has been the day of organisational change on UK railways:

1st January 1923 - the grouping; almost all rail companies merged into the "Big Four" http://www.passenger.chat/25835

1st January 1948 - nationalisation with the big four becoming "British Railways" - https://en.wikipedia.org/wiki/Transport_Act_1947

The NOT 1st January - (more done 1st April 1994) - Privatisation - "British Rail was split into about 100 companies, almost all of which were sold to the private sector or closed down." - https://researchbriefings.files.parliament.uk/documents/SN01157/SN01157.pdf

Edit to add - All three acts also archived for members
http://www.passenger.chat/mirror/HMG_Act_Reg1921.pdf
http://www.passenger.chat/mirror/HMG_Act1947.pdf
http://www.passenger.chat/mirror/HMG_Act001_1993.pdf