I am thinking of three particularly feeble, puerile terms that have achieved popularity:
Muppet
Cockwomble
Numpty
(I have neither any idea of the etymology of the second and third, nor any curiosity regarding the matter).
The supposed insult 'prat' was regularly bandied about when I was younger. The use of this insipidity has fortunately declined; the above trio are, however, worse.
The apparent lack of capacity for the modern British to compose a decent insult is only one symptom of Western decline.