In 1960 I would have said 100%. Nowadays we even have Scots MP's running England and when you add all the EU migrants and other asylum seekers, plus the so called English citizens that you see on TV that need an interpreter because they can't speak English...............need I say more ??
Depends how far back you want to go. As well as Celtic, Pict, Welsh, Irish, Scottish, Germanic there's a bit of Swedish, Danish, Norwegian, French, Italian et al.
Multi-culturalism is, and always will be what makes England English.
I am British, I feel British, I prefer to call myself British rather than English, and England has got various ethnic and linguistic minorities in it. None of those facts negate or diminish in any way the fact that England is by definition 100% English. If you want to ask "How many people in England speak English / were born in England / call themselves English" etc, then that is a different question.
Unfortunately, he never made it 'home' to ENGLAND, as he was killed in action & is buried in Ypres.
My other Grandfather fought for his country in WW2 & although badly injured, he did make it back to ENGLAND.
P.S. I don't think I'd have time to join the Army these days - I've been far too busy with in my life, bringing up two daughters (along with my husband), working hard achieving a successful career & now enjoying more leisure time, especially with my Grandchildren.