Exactly when it started life is a bit of an enigma (to me at least).  I have had sight of price lists and an incomplete set of both Trix/Minitrix and Hornby Minitrix catalogues.  Mistakes, inconsistencies and differences in various model names and numbering make research a little difficult - not to mention the changes of ownership of companies in the 1970s.  However here is what I have found:

A Britannia class loco makes its first appearance in a Trix catalogue in 1971 - shown here on the right.  It bears only a passing resemblance to the n-gauge Britannia that we know -  in particular the bogie wheels are spoked and the valve gear is far too complex for the n gauge models of the time. Further research suggest that this image might be a photo of Trix's OO version of Britannia, to which it bears a striking resemblance.   Whatever, this might suggest that at the time the 1970 catalogue went to print, there was no photo of the n-gauge model available - but of course, we have no way of saying that this was definitely the case.

The Minitrix Britannia does appear in the Trix 1971/72 catalogue with a picture which more closely resembles the loco that was finally produced.

The Britannia was Hornby Minitrix's only 4-6-2 loco until the Mallard came along in 1980.  The early Britannia had distinctive disc bogey wheels and had metal sliding components on the valve gear and had a metal crank pin on the centre drive wheel.  Although the Mallard and the early Britannia look very similar to each other, under the hood, they are quite different.  With the newly designed Mallard, the Britannia adopted the same centre chassis, axle and wheel set, bogies and the plastic square peg crank pin.  The technical data sheet that came with my brand new Mallard in 1980 also lists the New style Britannia with a variation of its original number ie 51-2037-10.  It does not show the Boadicea, Gresley or the two Flying Scotsmen.

• A pre-Hornby model with disc bogies and metal crank pin - Green & Yellow card/polystyrene box
• A Hornby Minitrix Britannia as above in a red & yellow card/polystyrene box
• A Hornby Minitrix Britannia with spoked wheels and plastic crank
• A Post Hornby Britannia as above with green & yellow plastic case

Around 1983, Hornby Minitrix marketed their second A4 - Sir Nigel Gresley which shared everything with the Mallard except for the red wheels and the Blue body shell.  Around 1985, the second Britannia class loco was released - Boadicea which was identical to the later Britannia, and I have seen it referred to as 51-2037-50, but this was soon changed to 51-2042-00.  I guess that this was about the time that the boxes and data sheets ceased to show the full 8 digit number.