Hornby and Trix used the 3-rail system until the late 50's or early 60's - the mid to late 50's is correct for the 80054 tank loco and also for the "Duchess" pacific. I cannot make out the name on the pacific so cannot give a date for production but it was made for many years. Probably "Duchess of Montrose" which was a 50's loco .
Hornby Dublo always used DC supply for 3-rail and their 2-rail systems.
00 scale was used in Britain because the locomotives if produced for H0 scale were too small for the motors available at the time so the scale was increased to 4mm/foot but because British locos are smaller than other locos they do not look much bigger than the equivalent H0 scale loco.