what will the police do if you dont claim modifications on your insurance?

It depends on the insurer. Some insurers have stopped insuring any modified cars. Some will accept them at an increased premium. Some will accept them regardless.

What the police will ask the insurer is would you have honoured any claim if this person had declared the modifications. If the insurer says no, or says that it would have affected the price, the police can charge him with driving without insurance which can result in 6 points and a £200 fine.

It doesn't stop there. When the insurer voids his insurance, it will be logged on a central database which all insurers can access. When he applies for insurance, the new insurer will ask if he has ever been refused insurance. He will need to say yes or the new insurance will be void. When he says yes, most insurers will refuse to take him on. He has effectively obtained insurance by deception and no insurer wants to take on someone who has been proved to be dishonest. If he can get insurance as a new driver with 6 points, it will be significantly more expensive than it currently is.

In addition, all insurance fraud, no matter how minor gets passed on to IFED which is a new police department specialising in insurance fraud. The don't act on every case as the more serious cases get prioritised, but he could potentially face fraud charges if they did choose to follow it up.

As you say, he will get his licence revoked under the new drivers act, but as a new driver, he should be up to date with the highway code, so he should be aware of the insurance regulations.

It means the policy was invalid from whenever the modifications were made.

He could get done for driving without valid insurance.

A loose battery is not likely to fall out, but if it can move about it can easily wear through the casing or crack during a sudden start or stop, dumping the acid out and possibly causing a short circuit & fire.

This is going to depend on how far the Police decide to push this as if they decide to get written confirmation from his insurers that modifications make it invalid they can prosecute him. No one but the Police will know this.

If the insurance is cancelled he may well have difficulty or it may be impossible for him to get insured by anyone for anything, not just cars, for the next 5 years or more.

Lies to an insurer often end in tears.

Its very simple if the vehicle had these modifications and he did not tell the insurance company his insurance is valid all companies ask this question and state if any modifications are made to the vehicle they must be informed so technically he has no insurance unless he has informed the insurance company, they wont honour his policy thats why you need to tell them if you modify a vehicle in anyway

He will only get done if he is caught by the police without insurance. The insurance company might give your friend the opportunity to make adjustments. He could always insure though another company .... or change his car if this is going to be too expensive.