I too miss Schweppes bitter lemon. However, a couple of drops of lemon extract in the tonic will do the job. I also miss the mint bush that grew outside my door on Long Island 40 years ago; a leaf from it added a grace to a gin & tonic.

A further poetic appreciation:

Her father's euonymus shines as we walk,
And swing past the summer-house, buried in talk,
And cool the verandah that welcomes us in
To the six-o'clock news and a lime-juice and gin. -- John Betjeman

I used to have a gin & tonic buddy who by chance once brought some limes; I took the opportunity.