Dark Ops and Coffee Boss 100ml. Frontline is a UK-based vape juice brand specialising in great-tasting, affordable E-Liquids. Combining an array of fruity flavours with a cool, menthol twist, this delicious line of juices is sure to satisfy even the most discerning sweet tooth. Look out for their COFFEE BOSS Brand for all coffee lovers. 100ml for only £15.99.