Fall back to cellular data when wifi is poor...?

I have both Wifi and Cellular Data (4G) permanently enabled on my iPhone 5; however, my iPhone will frequently latch one to free Wifi providers I subscribe to in preference to a strong 4G signal.

Is there any way to avoid this? At present, I can only disable Wifi entirely when this happens, but then when I get home and inevitably forget to re-enable it, I end up using my 4G data allowance without realising it. :-(

Additionally, when I get home, my iPhone frequently connects to a free Wifi provider in the area instead of my home network - is there a way to set the order of preference, like there is in OS X?