Thanks for all that Percy....you are right the Metric system came after the initial surveys made by General Roy and others.
Maybe we also owe an odd sort of gratitude to rebellious Scots for bringing about the serious surveying of the country?
Roy was also involved with the French when 'doing work' on the Anglo-French surveys.
The National Grid system plus the double letter system wasn't applied to our O.S. maps until the between wars years when the 500 kl, 100 kl grid squares and all the rest down to the 1 kilometer squares that are so important when using our maps today.
I guess that we appear to have kind-of got our metric maps by the back door when after the muddled (well to me they were) imperial/national grid maps that we both knew and used in earlier decades. When the first series of the 1:50,000 maps were introduced where the contours were directly converted from the imperial contour system...I still have some of these maps with the odd numbers showing along with the contours.
So maybe not until the second series of 1:50,000 maps came along did we have our fully metric maps.