Woodbridge school, Framlingham College with feeder prep, Gresham's with Feeder Prep, 2 x private schools in Ipswich - I think the girls school is going co-Ed. It depends where you want to live, you can live in Woodbridge & have an easy commute to London via Manningtree station.
Also good schools in Norwich and Bury St Edmunds. From 11 also Wymondham College, 1 of 36 state boarding schools in UK. I know you asked for independent schools but to include it helps you understand the options on the Norfolk/Suffolk boarder.