The posts on Pimms, I must admit, thoroughly confused me. Looking it up, Pimms No. 1 Cup is a gin-based bottled liquor, 25% alcohol by volume, with some liqueur, fruit juice mix and spices. Price in Canadian liquor stores is about $18.00.

From some of the posts I had guessed that it was a lemonade based soft drink.
It seems to me, if one wishes to drink gin, Bombay on the rocks, or a Travis McGee cocktail, would be a good substitute.

Travis McGee-
Pour a good dry sherry over ice. Shake slightly.
Decant the sherry down the drain.
Fill with good Bombay gin.
Enjoy!