hi,no a/c on but heater was blowing cool air ok. had done about 40 miles like this

hi,can anyone help me.r/rover p38 4.0 v8. driving along 2day,everything o.k when all of a sudden temp.gauge shot into red and car started to boil up.stopped engine immiediatley and lifted bonnet. coolant header tank boiling and escaping through overflow pipe.allowed all to cool and filled...