The vast majority of policies found on comparison websites are no good for modified cars. Most standard policies don't actually cover the value of the modifications. If you have a crash, they will only cover the cost of replacement with OEM components.
Admiral for example:
https://www.admiral.com/policyDocs/AD-003-023-Your-Car-Insurance-Guide.pdf
You will find that most "standard" policies have wording similar to this.
If its a modified car thats worth more than peanuts then you need like for like modification cover and agreed value.