Have tried one before, takes about 20 mins to boil 1 cup! As has already been explained, the maximum power you can pull through the fag lighter socket is crica 120 watts, that's 5%-10% of the heating power of your kettle at home.
I would suggest your options are:
1 - Small gas stove and kettle, need to stop & get out.
2 - Power Inverter connected directly to the battery and a low wattage (say 600w) travel kettle such as this :
https://www.ebay.co.uk/itm/Swan-SK19...QAAOSwSeVd6N1p
Choose an inverter with a capacity of at least x2 that of the load.
Clearly option 1 much cheaper than option 2.