White Horse History
The White Horse blend was created in 1861 by James Mackie (Mackie & Co.). In 1924, Mackie & Co. changed their name to White Horse Distillers; this is either due to the success of the brand or financial troubles. White Horse Distillers was acquired by DCL in 1927. One of the major malt components in the White Horse blend is Lagavulin, as is stated on today's back label.
As for a specific date for your bottling, you can find out the date/year the Queen appointed White Horse fairly easily, but this in not necessarily (and is probably unlikely) indicative of the bottling date. Especially given the size of bottle being a half pint.
Is it 200 ml? Here in the U.S. a half pint is 200 ml. This is a fairly modern size.