How about:

Ugly as a hat full of arseholes

Face like a smacked arse

Thicker than a frozen dog turd (sorry, can't bet my brain above that level, got a job interview in 3 hours, and I'm wasting valuable time on this!!)

Quicker than a trick on overtime (sorry - trick = client of a prostitute, overtime = prostitute whose rates increase after a certain length of time)

Subtle as a Sherman tank with the handbrake off

Personality of a litter tray/kitty litter

A useful as a cat flap on a submarine

As bright as an AppleMac technician (sorry, but it's what he said....)

As popular as Betamax video (a strange, little used format for videos, introduced as a competitor for VHS!!!)

About as busy as the Popes' pecker (again, from a catholic, I just pass 'em on)

As queer as a bottle of crisps

As much use as a eunuch in a harem

A former colleague of mine was describing another, not known for rushing to help, with the phrase: I've known tectonic plates move faster.....

LTS