I've been with co-op for 18 years now. Every year I try and find cheaper, and every year I can't.
So much so that in one year, I referred so many people that I got free insurance one year, and £150 off the following year. (That was when I went through an agent)
Currently paying £410 a year (with fees)
Slightly annoyed that insurance is going up so much. 4 years ago, my insurance for my Vitara was £113 for the year. The following year, £270.
They were also good when I got points 15 years ago. I was expecting a huge hike in my premium. But no, it went down £40
No idea what they are like with Modifications now, but when I had a 309gti lowered, chipped, and janspeed exhaust, my insurance wasn't effected. As long as I told them, it wont effect me unless I have a fault accident. But that was 14 years ago.