Chloe, firstly welcome to the SIDC. It's a great community, online and offline. When I was 20 I was paying out over a grand to insure something not nearly as exciting as an Impreza, so it might be a challenge. I was only 23 when I bought my 2005 WRX and people claimed I was mad for paying the insurance I was paying then, and indeed am paying now just a year later.
Having said that give this phone number a ring - 0208 6427868. It's a guy called Gary Moulson at Ketih Michaels Insurance. He's a fellow Impreza driver and might well be able to help you out. SIDC members get a discount , so you should mention you'd be joining the club if you got a car. There's a few people on the forum insured through them and have got good prices.
Certainly can't do any harm.
See if you can beat my best quote though, again when I was 20 and looking at a MKIV Twin Turbo Supra. The AA Wanted £10K insurance. The car was only £11K or £12K!