Why is Ginger essence hard to get hold of?
I can see it on Amazon, Lakeland, and Ocado, but the major supermarkets don't list it on their websites (you have to be a little careful searching as it's also sold as a supplement/herbal medicine).

It's rarely necessary to use it in cooking over other forms of ginger, and I never have, which may explain why it's only available from specialist sources.  One time it is used is ginger icing; that's somewhere I've used the syrup from stem ginger in syrup.  This works in many sweet recipes using ginger in liquid form but also replaces some of the sugar and water.  It's therefore best for recipes that rely on feel or taste rather than accurately measured small quantities.

You can make your own from fresh root ginger.  The link gives two methods with different results, for the purpose of flavouring beer, but there's no reason why you couldn't use them in other cooking.  The strength may be different to commercial extracts.