Sixth form is a big deal. Will you be going to university next year? In the UK or the USA? Having lived in England a few wonderful times in my life (and having been there a few months ago), I can tell you that we share a lot of the same global preppy brands. The Lacoste crocodile is the original, and oldest preppy logo: those shirts also withstand the test of time. I wear some old Lacostes that are probably as old as you, or older! HOWEVER, don’t even consider their extra-large crocodiles. Those are TTFW (too tacky for words). Same with the Polo pony. Only the small one will do. Tommy Hilfiger’s flag logo is small and tasteful, as is the small blue whale of Vineyard Vines (though I don’t know if that line is available in England). Etro is a good European line, as is Jack Wills, obviously.
Hope this is a good start. I’ll think of formal/office wear tips for the next time.