I can define the English quite clearly.
The English are the historic people formed in England through a particular ethnogenesis: the interaction and consolidation of Anglo-Saxon, Brittonic, Norse, Norman and other elements into a distinct population, with its own inherited language, institutions, customs, historical memory and continuity across generations.
No other people on earth have that exact history of formation. The French do not. The Japanese do not. The Nigerians do not. The Scots do not. Each has its own ethnogenesis.
That particular history is what makes English ethnicity English.
So saying “everyone is mixed” proves nothing. Mixture is universal in the abstract; ethnogenesis is particular in reality. What matters is which peoples mixed, where, when, through what historical processes, and what continuing people emerged from them.
Only the English descend from the specifically English ethnogenesis. That is precisely why “mixed” cannot be used as a magic word to dissolve the English into humanity in general.