For 1998-2000 model years (i.e. the last of the 'old shape'), a UK spec should be identified as a 'Subaru Impreza Turbo', and because of the differences in spec/value etc will probably say "UK spec" or "genuine UK car" somewhere in the ad.
Anything that age that's a WRX and/or STi is an import. If it's a Jap import, that means more power, but alot more pain.
There are alot of European imports on the market too. Though these were pretty much UK spec, they were 2 to 3 grand cheaper new, and that will be reflected in their used value (both to you and when you come to sell). Check if your local Scooby dealer is Euro friendly or not for servicing - apparently it varies!
Check the V5 and/or manual to find out the car's origin (I think sidc's faq lists relevant chassis numbers too). One way to be sure - anything on the forecourt of a Scooby dealer will be genuine UK. And expensive
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