Originally Posted by
orbitmic
Not that the UK is known for producing many of the oranges and lemons which are required for that very English drink...)
On the contrary, oranges and lemons have been grown in glasshouses in the UK since the 1700s. It is the origin of the term "orangery". This would have provided slices of orange and lemon in your gentleman's cocktail in a club (such as the CCR) before easy transport of imported fruit from southern Europe.