http://www.ebay.co.uk/itm/1pc-Prince-of-Wales-Commemorative-Coin-gift-/202031535182?hash=item2f0a04944e:g:g1kAAOSwaMpZnOtI
chinese are making better stuff than the royal mint is churning out lol
0.5 - 0.6mm thicker than what number? Without a thickness for the flan, you can't work out the volume and by extension the density. If you can calculate the density it will be obvious whether it is gold or not. I personally wouldn't hold my breath.
Don't forget chemical/acid testing is no-where near a reliable system. Plating, previous harsh cleaning, contamination, measurements or even a few specks of crap will skew the results.
Maybe something to do with the Empire. The ear on the Elephant suggests its an Indian Elephant. Indian troops were enlisted in the british army so I'd hazard a guess its probably the work of a bored indian solider. To some indians the elephant is a sacred animal. They worship them as one of their gods. (Ganesh)
The indians had a major rebellion in 1857 against british rule. So it could also be the 10 year anniversary of that. Quite often defacement of crown coins is done for political reasons.