Cordial is a thick juicy syrup added to drinks, but is not alcoholic itself.
I think the GP means ‘squash’ or ‘diluting juice’, which sometimes gets called cordial too (though I associate cordial with the thicker stuff)
It's worth noting that Ribena also comes ready made. If I ever buy Ribena, that's what I go for. Especially the carbonated version.
There are alternative blackcurrant cordials for diluting which are much cheaper and taste good. I think every British supermarket has their own brand one.