Q: as quite widely served in England during our brief summer (dunno about Wales and Scotland) the 'bottled liquor' you have identified is used as an ingredient in a long drink, the bulk of which is lemonade.
For some reason the long drink is known just as 'Pimms', presumably because the liquor is not served (AFAIK) as a drink on its own.

I suspect that caterers like it because they can charge cocktail prices for a rather diluted product, while reducing the number of obnoxious drunks they must humour. Serving by the jug also reduces staff costs.

As served among friends the recipe varies widely, as does strength, so it is wise to sample with caution.

Sorry if we have caused any cross-Atlantic confusion.