Here I am again, with a tale of doom

............
Firstly, let me state that I do not have any collusion with any insurance companies/brokers, other than in my own professional capacity (including Fraud, and, in a previous position, as a vehicle examiner for serious RTA's).
Secondly, i would always advocate total honesty in this venture - I pay no more for either car, with mods declared......
The blue one is on an agreed value, limited mileage, 'classic' policy and has modified exhaust, chip, roll bars. (KGM insurance through Adrian Flux)
The red one is on a standard policy (fully comp with M & S) - declared chip, exhaust, suspension, brakes, roll bars, interior.
Both cars for myself and Angie - SDP, commuting and business, kept on an open driveway, with Cat 1 alarms. It may help that we are both old (but not as old as Geoff, or looking as old as Dico

).
Paul Watkins also pays the same with mods declared as he did without (or he did last year!).
Okay, onto the bit no-one ever wants to hear or think about......
... Come the glorious day that your car gets nicked, the insurance company may use the excuse that all mods were not declared to refuse to pay out, or to reduce their liability.......obviously this only comes to light if the car is recovered, not if it is a total loss, but you will only get a 'book' settlement price. You can't really argue, otherwise they will tell you to sod off and seek legal advice - and I've yet to meet a 'no win - no fee' lawyer who would want to argue your case.
....next we have the even worse case scenario - what if your car is involved in a collision (not necessarily your fault) - the insurance company will have the vehicle inspected, and any mods MAY be noticed - there is a stock argument for 'improvements' to service items being okay without being declared (suspension components, brake components etc), as long as it doesn't compromise the safety or likelihood for theft of the vehicle.....
...after all, consider a bog standard Fiesta, Corsa, Saxo, 106 etc - if you take off the badges, tint the windows, fit big wheels, and drop the suspension a bit and don't tell your insurer that it is anything other than a 1.0 petrol, what happens when Jonny Druggie needs a fix - he'll take yours thinking that it is a Ghia Sport GSi and worth a few quid to Billy The Fence - he'll be a bit pissed off when he realises it has vinyl seats and a small engine, can't outrun a battenburg patterned Diesel Focus, and handles like a sponge cake - so goes to the local housing estate and makes a bonfire. (that is why insurers load the premium on a basic model with 'modifications' which may only be seen as 'accessories').
....even worse case, if involved in a serious collision (life changing or life threatening injuries as well as fatal), it will be examined by the insurers (who want to pay out less cash to injured people - after all, a bent car is worth a few grand - even if you write off a Veyron in doing so, it is "only" a million or so - but a life-changing disability could cost a heck off a lot more, so even a 1% shift in blame will save them a fortune)......
...it will also get looked at by VOSA examiners, and Old Bill - and I can assure you that 'modifications' WILL be found (maybe not all, maybe not every time, but they will be found more often than not).......
.....The insurers will still pay out to the injured party, but you get bu66er all - plus you get sued for everything you own - plus you get prosecuted for Fraud......
....and I've yet to meet anyone who has won a counter claim
Bottom line is that no-one goes out in the morning intending to have a crash, or have the car stolen - but they happen.
Also, what if VOSA/Police are doing 'spot checks' on the roadside - there is invariably a direct line (no pun intended) to the MIB and individual companies - anyone found with modifications is reported to the companies who invariably cancel the policy and have no legal requirement to refund the costs - and if you are on a 'monthly payment scheme' you still have to keep up the payments after it's been canceled as it is a legally binding credit agreement
It's your choice - I don't risk it, but if you chose to do so, I can't stop you.
(or maybe I can
)