Not too sure mate, some Insurers require you to be atleast 21 to drive a car like the GTI.
I've just turned 21, with 1 years NCB and an IN10 Conviction and I'm paying £1,200 Fully Comp on a GT TDI 150PD (2002).
Nate

Turbo-Dynamics Stage2 Hybrid Turbo, Sachs Organic Clutch & Pressure Plate, Large Diamater Downpipe, Turbo-Back Milltek Exhaust (De-Cat), Allard EGR Delete, Uprated Injector Nozzles, Jabbasport Stage 2 Re-Map, 18" RS4 Alloys Wrapped in 225/40 Goodyears, 312mm Audi TT Brake Conversion, Lowered -30mm on Eibach Springs, Forge Adjustable Quick Shifters, Aluminum R32 Interior, Cruise Control, 7" JVC Double DIN DVD Unit, JVC 600W 4 Channel Amp, Aluminum Bee Sting Aerial.
223BHP @ 4000RPM - 369lb/ft Torque
Build Up Thread Here.