I can't find it in the catalogue either, though that's not too surprising as coins issued by private mints (I believe yours will be The Commonwealth Mint) are generally issued in much smaller runs and at a very frequent rate. That makes it quite difficult for Numista members to keep up, and there's usually only a very tiny amount of collectors. The catalogue is usually most useful for coins issued by the national mint.
In terms of value, the spot price of 22ct gold is currently (12th July) ~£43 per gram, and your COA states it is 16 grams, so the direct bullion value is ~£688 today.
Obviously, a dealer will offer you less than spot price (as they want to make money on it), and if you sell it via an auctioneer (or privately on an auction site), they'll take a cut.
You might get more if you can find a collector for it, but these kind of things are a very niche market so it might take a while to find a buyer.
I hope that at least gives you a ballpark figure.
