In a modern house, the walls are usually stud type, i.e. not
load-bearing. So how strong are the ceiling joists in the loft
in a modern house?

Frequently, even in modern houses, there is a masonry wall in the middle
that rises to loft level (there was in my last house). Also, don't assume
that stud walls are non structural. Many houses are built entirely from
structural stud walls without any masonry in sight (except maybe a
decorative brick shell).

Christian.