I'll take a punt and then Colin can come along and tell me I'm wrong.
I would say 1914-1917. I base this on the spacing of the TT in BRITT. This appears to be the wider spacing, pictures on Colin's site here: http://www.aboutfarthings.co.uk/Farthing%20-%201915.html
I don't believe this obverse appears prior to 1914. Assuming 1918 to be too late for trench art, given the time it usually takes for the latest coins to appear in circulation (and also the time taken to do the work), I rule this date out, but it's not impossible by any means.
I covet it. It would make a nice present for a friend of mine who likes a bit of God.