It might not have been the point, but this excellently lucid answer is also a fine illustration of why short term interests are perhaps the biggest Achilles Heel of Capitalism.
Frankly, I think that if steam had lasted until 1980ish, the livery would have been reduced to unlined black with double arrows on tender/tank side. Why tart up something you intend to run down?
This is at least partly wrong. Hornby started to move production to China in 1995/96. Margate closed in 1999. The first new loco drive model was the Merchant Navy in 2000. The first loco drive 8F was in 2002. Lots of good information here http://www.hornbyguide.com/logo_menu.asp
I stand to be corrected by someone more knowledgeable, but I think the 1974 renumbering/reallocations were to do with the conversion of those vehicles for use in 4REP EMUs.