Have only just quickly scanned the thread but observe/comment:
1) A young provisional license holder has to be supervised = lower risk
2) A young full license holder is unsupervised = much higher risk
3) ANY MODS young/new drivers make (be they provisional or full license holders) will be *very very* heavily loaded. Insurance companies want to 1001% deter (by excessive premiums) all vehicle modifications. Skirts, dashing wheels, tinted windows, go faster stripes = BOY RACER. I mean no offence but this is how they work and often justifiably based on statistics.
4) WELL DONE on you. You have taken the due care / diligence to contact your insurer and ask/get quoted for mods. As much as you don't like the response I think you can now appreciate that had you put some trivial skirts on your car or changed the wheels AND NOT consulted/advised the insurance company then you would have been totally uninsured.
The bottom line to being uninsured is not just a criminal record but having to pay ££££££s for the care / death of one or more people for the rest of your working life!
If it is any consolation (I don't know your age) but come 21+ then things get a whole load better.
Both myself and my wife did motor sport (sprinting and track days) in our road going cars. We declared *EVERYTHING*. e.g. brake hoses, safety
stickers such as Ignition Off direction, yellow tape on battery negative lead, replacement of seat belt bots with "eye" bolts to take road legal safety harnesses, performance air filters (no other engine mods) and in my case a lower and stiffened suspension, free flow exhaust and wider profile tyres on alloy wheels (whopping 185s on my humble Uno SX).
We also submitted a planned "upgrade/change" list for approval (e.g. plumbed fire extinguisher, external emergency cut-off, and other mainly safety related stuff.
No problems what so ever and no premium increases.
The real ironic thing was one day a builders vehicle lost control and drove across our front garden and drive hit my Uno (which fortunately deflected the van from hitting my wife's Strada Abarth). Anyway an insurance company appointed independent insurance assessor came round. He tested the brake pedel, handbrake, tyre tread depths etc. etc. A long and detailed check list. When I asked why as the vehicle is OFF the road on my private property and does NOT have to be road legal for this incident he basically said I has a detailed check list to work to and this is a "vehicle state", "modifications" etc. etc. What the insurance companies do with the report is nothing to do with him.
I also remembering him saying that were this a Ford Cosworth/similar performance and engine management tuneable car he would have would have connected to the ECU and checked the ECU map checksum/image for changes.
People (all), please make no mistake. These insurance companies are commercial concerns with profit, viability and employees to support. If there is anything they can wriggle out of or reduce pay-out on then they will. You would do the same.
*uck with them and they will *uck you over even more. Be honest and open with them and then if you have made a genuine oversight then there is unlikely to be a problem and they are more likely to want to advertise their behaviour to genuine errors.