If it doesn't charge £15,000 a year then most posters on this website are not interested.
The results at La Retraite have improved significantly in the past three or four years, even though the intake has remained largely the same. It's definitely on the up but still behind other Catholic girls secondary alternatives such as Ursuline, St Phil's and Sacred Heart. If you're not Catholic I'd be doubtful if you'd get a place. If you are Catholic I'd wonder why you aren't at least considering some of the others. St Phil's seems like a long way but (if Southern works!) it is not so far by train.
Burntwood is good - certainly the best non-Catholic Wandsworth/Lambeth secondary choice for girls after Graveney, but you do have to wonder with the selective places if it shouldn't be getting better results. It's got a shiny new building but they don't impress me. It's also not so easy to access by public transport.