additional products recieve discounted shipping?

The shipping methods are global to orders.
So for example, you could do that:
set a weight of 1kg for all your products.
Create shipping methods like this:
from 0 to 1.5kg => £3
from 1.5 to 2.5 => £3.5
from 2.5 to 3.5 => £4
...etc...
and that will do what you want.