Yes, you can raise a complaint with them and then escalate to the financial ombudsman. Requote yourself again via the same website and same insurer, this time ticking the right box, and see if they are willing to provide cover. If they do, do a screen print as evidence and save it. This shows they should not have voided the policy, but amended and charged you extra. The only time a policy can be cancelled is when it's fraudulent or wilful misrepresentation, where the insurer would not have provided cover had it have known otherwise. If you can prove they would have, then they will have to reinstate the policy whilst ensuring you are not negatively affected in any way.