Double glazing - thin frames?

[Mosling]

Our kitchen window is large (c. 1.5m X 1.5m) and overlooks trees. We spend lots of time looking out of it, but it needs replacing. It's 1990s aluminium, badly fitted, broken hinges, mouldy frames, scratched panes. But uPVC frames seem really intrusive and the well recommended local company insisted we need more than one pane with huge thick white dividing bars (it's a first floor flat in Scotland if this matters).

Now, houses on Grand Designs do not have this problem, they have vast acres of glass all over the place. What do I need to Google? Is more aluminium the answer and what's the cost difference?

The other windows would all be okay with uPVC and we're not loaded (average salaries, 3 days childcare) but want to get it right if we can. It's a c1900 brick flat so no conservation issues.

Does timber or composite have a thinner frame? Is there a wonder material I'm unaware of? Or a way of getting larger glass units into uPVC? Enlighten me, oh Mumsnet...