Complex sentence: She completed sixth form at the independent Westminster School in London, having earlier left an all-girls Roman Catholic school to study at Westminster because she felt it would give her a better chance of attending the University of Oxford like her father.
Split into simpler sentences: She completed sixth form at the independent Westminster School in London. Lightwood had earlier left an all-girls Roman Catholic school to study at Westminster as she felt it gave her a better chance of attending the University of Oxford like her father.