Oi'm wid' dem

. The main attraction of the 500-700 series are a better quality flute that is reliable and is easy on the purse. However, it is still designed to be not too hard to play, which means that some "choices" about tone production are not left up to the player, but are limited by the cut or shape or position of holes (both key holes and lip-plate hole, but most especially the lip-plate tone hole).
You should at least try a different headjoint, preferably taking your time over it, because a headjoint that offers you more choices also makes the work harder for you until you are used to it. The 600 and 700 bodies are nice, reliable bodies with a kind of boy-next-door security about them

, so please do try them, but if I were you and if I were very rich, and talented enough to be thinking about study at university level, I'd be looking at a headjoint that can offer you more.
The best of the 700 (silver body, silver keys) models are getting close to the price of the all-handmade 800 series, and there is a kind of "crossover" 711/781 series which is basically an economy 800 series flute with an unusual advanced EC-light headjoint, but I don't know if it's in the UK, and even if it is, it's in price range where you should be looking at other flute brands and only choosing the Yamaha if you like it best, not because it's cheapest.
You may want a bright, carrying sound for solo work, or you may want something with lots of bang for orchestral work - each flute maker has a characteristic sound, and the best-regarded flute brands don't just cost more because they're "better", but because they offer many more variations of design and material at each level, whereas Yamaha only offers a few models, which saves a lot of money both in design/research and in manufacturing.
You should remember that the handmade Yamaha 800 flutes are made in a different factory, by different people, so you are not just getting "more handmade work", you are essentially getting a different brand of flute. I haven't played one, so I couldn't even tell you how they compare to other Yamahas!
The 500-700 series all share the same body design and headjoint. 600 series - silver head with silver nickel body and keys, 700 silver head and body with silver-nickel keys, 700 silver head and body with silver keys.
They all have drawn tone holes (pulled up out of the metal the flute body is made of, not stuck on separately - this makes the flute strong, because bad soldering is a problem, but the very "best" flutes are usually not made this way. However, the design of the body is no the same as the 200-400 series (though not dissimilar probably), and it has a lot more work done on it - annealing of the inside of the flute, hand-cut angle of each hole etc.
The headjoint is the "EC" type, which in Yamaha-speak means a high-set lip-plate/tone-hole to make life easy when playing low notes, and a "C" taper, which lies between the "crank up the voume" sharply stepped Y taper used in the beginner flutes, and the straight G taper used in *some* of the more advanced Yamaha headjoints so that you have more control over your sound (but you have to work harder, because there's also more resistance).
Long long long discussion on comparing low-end pro-model flutes!