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 an appearance in a Trix catalogue in 1971 - shown in the picture above.  It bears only passing resemblance to the n-gauge Britannia that we know -  in particular the bogie wheels are spoked in this model and the valve gear is far too complex for the n gauge models of the time.  Further research reveals a striking similarity between this picture and the pictures that can be located of Trix's OO gauge model of Britannia.

This might suggest that at the time that the Trix 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.  Neither do I know if the catalogue that I have seen is the only one produced in 1970.  It might not have been !

The model 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 bogie wheels and had metal sliding components on the valve gear and had metal crank pins on the centre drive wheel.  In 1983, Hornby Minitrix marketed their second A4 - Sir Nigel Gresley - which shared everything with the Mallard, but sported new red wheels and a blue body shell. 

Around the same time, Britannia class bodies were fitted to the same chassis as used by Mallard, and the new Britannia and Boadicea models were born.  Hornby marketed the Boadicea as N217.  The original Trix number for Britannia was 51-2037-00.  Initially, they seem to have assigned the new Britannia the number 51-2037-50, but changed it to 51-2042-00 (information taken from the technical sheets issued with the A3 and A4 locos).  The new Britannia was given the number 51-2037-10.  Since the boxes show only 5 digits, in effect it retained the original 12037 designation.

I do not know if Hornby marketted the new style Britannia as its original number of N203 or whether the new Britannia came after Hornby stopped their association with Minitrix.  A cursory look at completed listings of Britannia on ebay suggests that they did.

Year Photo	Model	Year Catalogue	Trix No.	Hornby No.	Colour	Picture / link	Model	Box
1971	Britannia	In Jan 74, not new). Appears as a pic in Trix 1970 catalogue, but valve gear is wrong. It looks very much like the Trix OO model.	51-2037-00	N203			Brittania 4-6-2 70000	Minitrix G/Y Styrene
1985 ??			51-2037-10		Green		Britannia 4-6-2 70000	Hornby Styrenes
1988			51-2949-00		BR Green		Flying Scotsman 4-6-2 60103	Minitrix Plastic
1988			51-2950-00		Apple Green		Flying Scotsman 4-6-2 4472	Minitrix Plastic

1973 Catalogue List