I tried just tried Googling 'ginger wine essence'. All of the references to the Friar Tuck's brand are historical, suggesting that it's not been around for many years.
However, there's an alternative brand available, with chemists and 'home brew' shops topping the list of likely suppliers. (It's a bit of a long shot but Boots used to do home brew stuff and, obviously, they're a chemist's so perhaps they might stock something similar to what you're seeking?)
Alternatively, buy online at one of these suppliers:
http://www.stonehelm.co.uk/WineSupplements.htm l
(See under 'Sinatin')
http://www.art-of-brewing.co.uk/acatalog/Dtl00 05.htm
(Listed as 'Sinatinsa')
http://www.brewingshop.co.uk/sess/utn;jsession id=154942f84414c70/shopdata/product_search.sho pscript?query=ginger
(Sinatin again. Click on the link for the recipe to make ginger wine).
Chris