I found two were abput the same, CIS and Admiral, CIS are like Liverpool Victoria with respect to mods, in that they dont charge any extra unless they effect performance, and wiht a full exhaust, de-cat and IK on a 1.8 RSi my premium stayed the same.
Admiral charge extra for mods.
Also depending on how long till you are 19, try HIC, they gave me a fairtly cheap quote, when i told them i was 19