As has been said previously i think anyone who does not declare mods is quite frankly a fcuking idiot.
Why give an insurer ammunition to not pay out
Even if your name begins with Sheikh i doubt anybody wants to be left without any money and a car.
What are you going to do if you crash the car and the insurance say "Sorry old bean, that ED30 engine you dumped in there and never told us about isn't what you were covered for!"
I can understand why youngsters might consider not insuring when the insurance starts costing more than the car itself. But if you can't afford the insurance, don't fcuking modify or buy the car in the first place!!!