British slang help needed

[Jim Ferer]

Tim:"[portacabins]...are the small cabins used mainly on building 
sites (and some schools, hostpitals, ...). They arrive on a truck and 
are intendied to be short term office space. (I don't know what a 
portacabin bed is.) What's a portacabin called in the States?"

There's no term for them here.  They're just "prefabricated metal 
buildings" or "prefab sheds."  "Portacabin" sounds to me like one of 
those brand names that became the name of the thing itself.  Are 
public address systems still called a "Tannoy" in Britain, for 
example? In the US, for many years any refrigerator was 
a "Frigidaire."