Also could we make a decent living out of this?
If you had done so 25 years ago, yes. But any need of this sort has long been met, even in places like Tuchola. A lot more native Poles have learned English to the point where they can teach it at all but the most advanced levels. This reduces the need for native English speakers. And in a community that size, the market for French will be a niche one that is also probably satisfied at the moment, as well.
I would say that it is unrealistic to expect that you will be able to support yourself and your family by teaching English or opening a school nowadays. You'd have to operate quite a successful operation, and that would take years to establish. And it might be impossible to do so in a town as small as Tuchola. Until then, you would have to live on savings, hoping that one day, your investment would pay off. And probably hoping against hope.
Frankly, I think your wife's idea of moving you all to Poland is totally unworkable. It would be far more feasible to continue living and working in the UK and taking extended vacations in Poland. There is just no way that you will be able to avoid a drastic cut in earnings and savings potential if you move to Poland. And, like I said, you'd probably have to live off of savings for an indeterminate number of years. Basic math is your enemy here.