Which country colonised England?

The use of ‘country’ here is problematic. When England was substantially colonised (quite some time before it was ‘England’, in fact), the modern conception of nation-states did not exist. The colonists themselves were not representing anything we would recognise as a ‘country’, so the term cannot be applied accurately.

Prehistoric colonists remain largely unknown, to the best of my knowledge. Archaeology demonstrates that migration was a factor even in neolithic times, but we can’t know the extent of it. Celtic culture certainly arrived from somewhere, at some point, but when and whence are unknown and possibly unknowable.

The Romans conquered the British mainland south of the Hadrian/Antonine Walls, and colonies were founded during the period of their rule.

After the Roman Empire withdrew, invaders/colonists came from all over northern Europe (mainly the coastal regions of modern Germany, Netherlands, Denmark, Norway, Sweden and others - but before any of these places were organised into ‘countries’).

The most recent were the Normans who followed William the Conqueror and his successors. The Normans had more of a national identity, but calling Normandy a ‘country’ would be a stretch.