Your living room counts as well, your kitchen also if it is large. Assuming your kitchen is small, you have a 3 roomed flat.
3 rooms can accommodate 5 people.
Children under 1 don't count, and between 1 and 10 they are a half.
It is not considered to be overcrowding if it is due to natural growth, children getting older.
http://england.shelter.org.uk/get_advice/repai rs_and_bad_conditions/overcrowding
You could rent somewhere privately, or find a swap.