The Book of Invasions might interest you. It's a collection of medieval writings which purports to give the history of the founding of Ireland; basically it's an Irish creation myth. Some of it is relevant to the creation myths of other parts of the British isles, for example the character Scota, an Egyptian princess who settled in Ireland, and from whom, as legend has it, the name of Scotland is derived.

Bede's Ecclesiastical History (8th century) is another very important one. Bede's account of the Anglo-Saxon invasions forms the basis (along with the Anglo-Saxon Chronicle, which used Bede as one of its sources) of the 19th century narrative (which is still taught in most surveys today) that the Romano-British were driven out of England by invading Germanic armies in the 5th century.

For some critical discussions of Bede's work (if you're interested, or if your assignment requires any secondary research), see W. Goffart, The Narrators of Barbarian History, and G. Halsall, Worlds of Arthur.

Have you considered Bede's Historia Ecclesiastica Gentis Anglorum (Ecclesiastical History of the English People) who heavily influenced and was used a secondary source by William of Malmesbury?

Also just to clarify, do you mean origination myths of the peoples who inhabited the British Isles? If so you you could perhaps use Roman histories of British tribes pre-Roman conquest and maybe even the origin myths of the later Normans who later of course became part of a Anglo-Norman society after the Norman Conquest.