What engine modifications would you consider illegal? As long as my car passes e-test am I safe from eco police?. Is lowering your car illegal?.

I know there is an HTA for this but I do not recall the number.

It basically states that you cannot modify your car in a way that would make it unsafe to you, your passengers and to the public at large.

Nitrous is now illegal to have on a licensed car. as I understand it, if you have NOS, the bottle has to be disconnected if you are driving on the road and highways.

But then, it's only an issue if you get caught.

Other than this, you should be good to go.