all the insurance companies said my car was 125 bhp as well (mines def a turbo) i just entered the car details, not the reg, they rang me and i explained and they sorted it straight out, still bumped up my premium tho 
Is very strange. I spent ages last night trying around 20 different 1.8t listed cars, from on here and ebay and all of them came back as the correct 150 bhp, it was only the one I am buying that came back as the 125 bhp, although on HPI it says it is a turbo.
I have asked him for a picture of the engine, so am waiting to see one of those before I go to him with cash. But yes, def need a picture as need to insure it as a turbo , if indeed it is, as like you say, the premium is more.