Admiral were cheapest for me. 23 years old, 0 NCB, living in central london, im paying £1000/year.
I find insurance companies care a lot about engine size, Adrian Flux laughed at me and said i had no chance of getting insurance on a 3.5l car because i had 0 NCB. The quote ended up being £3000. I asked him for suggestions, and he said a 1.0l Vauxhall Corsa. Not really sure what they 'specialise' in.