Insurance works this way (in a nutshell):
The insurer evaluates the risk.
The insurer then calculates the Premium cost based on statistical analysis of the risk involved.
If you do not disclose material information to the insurer, they are unable to correctly assess the risk. Your Policy will be void because you paid the wrong amount. They might even go as far as refund you what you have paid on the premise that the amount was not sufficient to buy you insurance in the first place.
More specifically, in the insurer's eyes, anyone who modified their car so that video can be watched while driving, poses a higher risk in general. And by not disclosing this information to the insurer, that person is preventing them from correctly assessing the risk in providing insurance (or even refusing insurance altogether). This voids the policy on the premise that the correct amount was never paid in the first place.
It is no different to walking into a coffee shop, picking up a £2 sandwich and leaving £1 at the counter. Your £1 did not buy you half a sandwich... the staff will likely hand you back your £1 and take away the sandwich saying it was never yours because you did not pay the full amount for it.
The insurer is also backed by criminal legislation - in extreme cases you could be prosecuted for either driving while uninsured, or worse still obtaining insurance by deception.
The main reason that this issue is not taken seriously by many people is that unless there's a claim against the policy, no one will ever know, and because on most days most people do not have claims made against their policy, very few people actually get caught.
