You WILL have to re-wire for 12V.
...if you need > 200W power.
Our house wiring (230V) uses 1.5 mm² and 16A fuses, so I'd set my limit to rather less than majenko's 20 A per wire.
If you just want to power a 12V TV and eventually 3 * 20W halogen bulbs on one wire, I don't see a problem.
With 2.5 mm² wiring, your limit might be higher, but a water pump or your "Power Tools" (?) running at 12 V might be something different.
Also check the switches for their current rating.
Perhaps you feed the battery power into your existing 16A wiring, but have a more powerful 220V supply by your generator, if you temporarily need more power ?
Or the generator feeds 220V into that existing circuit, and you add a separate 12V wiring ( with different plugs and outlets, of course )