So arms exports are like any other and selling them simply 'creates British jobs'. Does that mean everyone has to suck it up when these weapons may be used right back. Not important what hands they end up in, they created jobs. It doesn't matter who they kill.
Also, did the US and UK governments arm Daesh/is when they thought the Arab Spring could overthrow Assad? Along with the other anti Assad government groups who are not a million miles away from Daesh/Is.