go with bell, but make sure you do it online, not over the phone. my m8 is 22 and tried it but they didnt let him (over the phone). im 19 (18 when i took out the insurance), and i got it online and it was an option, only an extra £30 or so. i have been pulled by the police and had to produce my insurance at the station to prove it (as police didnt believe me, said i had to be 25). Anyway, it was all good, all legit.
Its awesome, and i am going to miss it when i change insurers.
p.s. changing insurer due to my conversion, not because i wasnt happy with bell.