The 1.2TSI in the ST is approx. £1300 more than the 1.4! Take for example the current cost of the 1.6 upgrade from the 1.4 on the SC which is only £600, should make the 1.2TSI SC around £12900 which still makes it cheap compared with the Polo SEL 1.2TSI @ £14700
I'd assume the car will also come with a 6spd manual, and should be easier to drive than my 1.6, as it has more torque. Tuning potential it should be good for 130BHP I'd guess. The engine isn't supercharged, so far as I'm aware.
Definitely wish this engine was available when I'd ordered mine, if only for the lacklustre fuel economy of the 1.6. Hell, even Skoda blagged this engine before SEAT!