Hi Anna,
First of all, what does your teacher think about you upgrading? He or she will be in the best position to advise, as s/he regularly hears you play on your Yamaha. If your current flute is in good order, even if its fairly old, it should take you through to Grade 8.
If you do want to upgrade, you need to think about how much you can afford to spend. Again, if the Yamaha is in good working order, upgrading the headjoint is a possibility, and a cost-effective option. A word of warning, however. Don't assume that because a headjoint is solid silver it will automatically be better than the one you play on at the moment. A lot depends on the manufacture (whether hand-made or factory-made) rather than the material. Some of the best step-up flutes (eg Miyazawa 101 and Altus 807) are silver-plated.
Probably the best advice I can give is to go to a flute specialist shop and try out different flutes and headjoints. If your teacher is able and willing to go with you to listen to how you sound so much the better. If not, then see if you can borrow a mini-disc player and microphone and record yourself playing on the different instruments. You can play the recording to different people, including your teacher, and see if they agree on the best sound.
Good luck with your flute-playing and research.