You are an Englishman I suppose....therefore your basic bread is not
sweet and your normal doughs are less rich than the North American
version.

> always follow the instructions to the letter. Kneading takes 10 minutes,

If the dough appears dense, it can be it needs s more kneading /dough
development and also needs more fermentation. before you divide it ,
mold and proof.
You are using a strong flour and that animal kneads more" rough
treatment" or kneading to attain the desired dough development.

> I wonder if a *second* kneading, or
>something, would result in a lighter product?

certainly

>Perhaps adding some ingredient that's not mentioned in the
>instructions, like sugar?

The Brits are not known to add sugar into their basic doughs.
Besides English flour are known for its high diastatic activity( can
provide sufficient sugars for the yeast to "munch " on.)
Just give it a proper kneading,check for' window pane' and give it
prolonged fermentation with knock down in between to allow the gluten
to stretched to the limit and hence will result in better volume and
lighter textured bread.