A small quantity of hops, (
There are several varieties commonly in use in the UK, each of which has it's own aroma and bitterness.
During the boil the alpha acid content of the hops emparts bitterness to the beer. The essential oils that provide an aroma will boil off.
"Irish Moss", which is in fact dried seaweed may also be pitched in at this point to assist with coagulation of the Trub i.e. the gunjey bits you donít want, in the boil.