Perhaps how literary attitudes towards immigrants/ invaders have changed over time - pick a few texts to compare, from when the Norse/ Anglo Saxons were moving here, and from modern times, all of which have a focus on attitudes towards the immigrants. You could also look at texts from the time in between - it might be interesting to look at the attitudes towards 'black' immigrants shortly after the end of the slave trade, and compare it to the attitudes today. This would obviously have links to politics as well as English, and perhaps a bit of philosophy...
There could be all sorts of interesting things to compare - e.g. the adjectives used to describe them, the actions described towards them, their apparent position in society, etc.