What school do you go to now? And what do you want to do at A-Level? Finally, what are your predicted grades? Personally, I would not advise retaking all the subjects you don't get A*s in. It is a lot more work.
For Tiffin, you will need at least A's in the core subjects and A's, preferably A*'s, in the subjects you wish to take for A-Level. Regarding the Bio, Chem and Physics tests, I would not retake them yet. You may get A* in Unit 2 and 3 which will bring the overall grade to an A*. Also, the ISA count for a large part of the final GCSE grade and they are relatively easy.
Also, the application to Tiffin would be on the basis of your predicted grades not actual grades. So, if you get an offer you will need to meet that.