A little late to this thread but for future searchers I am also with A Plan and have been for several years. The first year I repatriated from USA and imported my lightly modded C6 I was with Flux. I shouldn't complain as they were very helpful with insuring it from landing on VIN only etc whilst it was away being converted/MOT'd. However come renewal they tried every trick to up my premium and I was nonplussed to say the least.
Shopped around a bit and found A plan to be superb from a customer service perspective. A great price too, along with track day cover and no issue with mods, I have remained loyal since. It tends to go down about a tenner a year and I have to admit I haven't shopped around as I can't imagine anywhere else being that much cheaper as it to be worth the time spent on the phone explaining all about mods/US NCD etc.
They cover our other two cars as well.