Probably cheaper to buy the Gin in Tokyo instead of duty free store which saves you lugging it around on the plane, etc
Just looking at BIC Camera prices as example
UK Bombay Sapphire Gin 750ml 1,580 yen
Plymouth Gin 700ml 1,480
Beefeater 898 yen 700ML
and others.
http://www.biccamera.com/bicbic/jsp/..._PAGE=1&PAGE=2
Even convenience stores often sell bottles of gin (lower end stuff )