Instructions clearly say “report your result”, not “report result only if positive”. But it could definitely be stated more clearly “report the result even if it is negative”.
Having just done it for my household I agree it could and should be made quicker and easier. Even a button saying “report another member of the same household” to save you retyping the address would help. And scanning straight from the NHS Covid app would seem like a no brained.
Positivity rate (ie number of positive tests/total tests taken) is a really useful piece of statistical data that enables us to keep track of the progress of an epidemic without being skewed by the effect of surge testing. If LFTs are going to be part of the country’s future plans then encouraging all results to be recorded and making that as seamless as possible would make them much more useful.