Sorry if you don't like the numbers, but that's just is the way it is.
Indeed, flight instruction is not a justifiable investment, or we'd all be on dragon's den offering a decent 3 year return.
It just doesn't work that way. People do it for the passion, for the fun of it, and for the hope that it will one day lead to something that might pay the bills. It's only ever a hope, and usually involves sacrificing everything and putting a huge risk on financial stability.
I've personally lost two girlfriends and I've known one guy lose a wife because of instructing, but I wouldn't change it for the world.
You pay £30k to earn £15k a year, it's as simple as that. If you can't make those figures work for you, fair enough.
Full time instructing means committing to 6 days a week, typically 8 hours a day. As has been stated above, you only get paid the hours you fly, so an absolutely full day would be perhaps 5 lessons with at least an hour unpaid ground work for every hour flown. A more typical day is 2 hours flown, 2 hours unpaid ground work, and of those 6 days a week at least two lost to bad weather, when it gets really bad all 6 are lost to bad weather, then you're really in trouble if you can't negotiate a retainer.
The entire industry is based on who you know, so if you make best friends with a flying school owner (which from your questions I would assume you have no contacts at all at the moment) then you might be ok, but it's all about being in the right place at the right time with the right experience.