What criteria would you assess to gauge whether to quit a Ph.D for a job in industry?
"you think" you'd be happy working might not be enough, depend on the situation I suggest you to take a leave of absence (usually universities in the UK allow this) for 6-12 months, and get a job so you know for sure what ticks for you, then decide whether to return to your program or keep the industry position. Don't feel guilty for taking options that are readily available. 

I had similar experience and this, to go or to stay, might be the hardest question I had to answer myself for the longest time. In the end I decided to stick it out, but I also have a close friend who's having the time of their live ditching their PhD 2 years in. You just need to work out the best approach for your situation, and the best way to do it is from a little bit of distance. 

Good luck!