If an insurance company finds that you've failed to provide full information when seeking cover, they might report the matter to the police. 'Attempting to gain a pecuniary advantage by deception' is an offence under Section 16 of the Theft Act 1968, which can result in up to 10 years imprisonment. (You'd also find it extremely difficult to get any form of insurance in future. Additionally, most banks refuse to allow anyone with a fraud conviction to have an account with them).
If you did manage to get insurance by keeping quiet about the accident, the insurance company might carry out further investigations if you made a claim. As well as the possibility of criminal action (as above), the insurers would refuse to pay out, leaving you to pay any claims from the other driver. (If someone suffered injuries, from an accident caused by a driver without valid insurance, resulting in them requiring life-long care, the other driver would be personally liable for paying a multi-million pound bill).
Chris