It will very much depend on what you modify. If you only drive on your own land, that's mostly fine (possibly not fine if your own land is a road accessible to the public).
What will cause you problems:
The manufacturer has a general permission to distribute car models with certain characteristics in your country. There's trouble if your modification means your modified car doesn't have these characteristics. Your car insurance is for a certain car model.
Your insurance may be invalid if you modify your car. As "boy racers" were mentioned, part of the insurance cost is based on the type of drivers this car model attracts. If you change it from "boring old family car driven by boring old dads who never have accidents" to "exciting boy racer car driven by maniacs who crash their car regularly", you would be expected to tell your insurance so they can adapt their insurance.
As far as passing your MOT test goes: If you don't advise the tester of the change, and therefore the fact that your car is unsafe is missed, your MOT is not valid. Same in the time between making changes and the next MOT if your changes made the car unsafe. Usually that will be found out at the worst possible time: After you are involved in a costly accident.