She has dual citizenship. I would suggest the circumstances your grandparents found themselves were somewhat different than her parents.
The rules behind representing countries in sport etc has been a bone of contention for many years. Half of the English Cricket team had strong S African accents.
WTF she wasn't born here and neither of her parents have any British connection at all, she is not British. Are you Jack Charlton? At least the other great British Tennis player Greg Rusedski had an English mum.
WTF a buffoon being born anywhere has to do with it.