I dont think that is correct. If you put your charger on when on HU or start your engine you can get up to 14.8 volts, your 12 volt outlet sockets will carry this increased voltage. Thats why the inverter is best i think.
Comet Electricals have a good offer on at the moment 15" flat screen TV c/w Freeview, DVD drive, HD ready and 12v input All for £140!
For an extra £44 you can have five years guarantee new for old replacement.
Model: Kenmark LVD1586D It has all the usual features like auto tune.
I know three people who have bought - all delighted.
BTW I have no connection with Comet Electricals.
When I looked at Comet tonight, they have what I think may be the enhanced model to the above.
It's the Kenmark LVD1587D It's currently only available on-line.
Anyway I decided buy one, as it's more versatile, with less boxes, and wires all over the place. I now just need a decent quality voltage stabilizer as the company quoted in this thread is out of stock.