Archers schnapps comes in a variety of ABVs. If it's below 23% (some is only 18% ABV) it should be classed as a fortified wine, and sold as would port and sherry in 50ml measures. If it's ABV is higher than 23% (it's made with up to 40% ABV) then it's a spirit and should be sold in 25ml or 35ml measures according to whichever the individual pub provides.