May I have some informal or slang terms for such a person? (especially in BrE, please)
Many thanks in advance.
And, are the /p/ and /b/ sounds pronounced on numpty and numbty?
A waste of space- you could also use twit/prat/berk, though these have a less specific meaning.
You could also use "worthless" instead of "useless".
I'll offer "drongo" because although it's essentially Australian I know that it's had some currency in GB.