Here is my solution for this :
mkdir temp
mkdir results
cp /usr/share/dict/american-english ~/temp/american-english-dictionary
cp /usr/share/dict/british-english ~/temp/british-english-dictionary
cat ~/temp/american-english-dictionary | wc -l > ~/results/count-american-english-dictionary
cat ~/temp/british-english-dictionary | wc -l > ~/results/count-british-english-dictionary
grep -Fxf ~/temp/american-english-dictionary ~/temp/british-english-dictionary > ~/results/common-english
grep -Fxvf ~/results/common-english ~/temp/american-english-dictionary > ~/results/unique-american-english
grep -Fxvf ~/results/common-english ~/temp/british-english-dictionary > ~/results/unique-british-english