As always, it depends. The test is always one of reasonableness - could or should a layperson reasonably have known?
Information for financial businesses about how we deal with complaints about misrepresentation and non-disclosure.
www.financial-ombudsman.org.uk
The underpinning legislation basically says that if a reasonable consumer would have known (in this case about the modifications) then you've misrepresented things even if YOU didn't know. In other words, if you're stupid or ignorant it doesn't matter

In the case of your lowering kit, the courts would likely look at a photograph of your car vs a photograph of a standard car and assess if a layperson would have noticed the difference. Clearly engine mounts are unlikely to meet the bar but lowering or other visual mods might well do. Similarly with things like exhausts - completely obvious to a layperson if an exhaust modification has been made in most cases. In your specific case where you have knowingly misrepresented things then you're likely to come unstuck - they aren't stupid, they have engineers and other investigators specifically to find out things like this.
Outcome wise, if they can demonstrate that without the misrepresentation (ie everything presented exactly as it is in reality) they wouldn't have insured you they would be able to avoid the claim (save for anything 3rd party where payout is mandatory under the RTA, however they may well seek damages from you in that instance!). If they would simply have charged you more then they would be able to reduce the payout by the proportion of the new premium vs what you had paid eg if your premium was £1000 and the all-mods-declared premium would have been £2000 then they would only need to pay 50% of the claim.