Enjoy your posts. I am a Silver bull more so than Gold; however, you suggest that Silver is undervalued - I assume in the context of Gold. Value, apart from being in the eye of the beholder, in a truly free market is a demand / supply equation. Thus if Gold demand / supply stays the same but a new use (Refer Jan 17 ALTA Report on new electrics) is found for Silver (with possibly little change in Supply) then Silver should increase in value against Gold.
Sovereign / Half Crown comparisons are interesting but there is much more to the equation. One Pound Sterling was equal to one pound (physical weight) of Sterling Silver (92.5% pure (as you mentioned); however, the half crown should have been 2 ounces in weight (being 1/8th of a pound) yet it is only 0.45 oz. So the Crown (meaning King George) effectively stole money from commoners in 1918 by only minting a coin that was closer in value (compared to the Pound) to a Shilling.
Notwithstanding, after having regard to the Silver price GBP 13.44 / oz your purchase melt value was only about one GBP 1 over the Silver spot per ounce, which is pretty good for a coin (as compared to bullion).
Look forward to your next post.