No mate, you only have to tell the insurance all the mods.
As for the car certificate, or the V5 as it's known in the UK, you only have to update that if you change the colour of the car or dramatically modify the car, ie change the number of doors or wheels!
They're not interested in things like modifications, it's a lot easier when you're over here mate, don't have to worry about TUV or anything like that :winky:
thanks Aido :) Indeed, it makes it much easier to modify cars. Unfortunately as I have a foreign driving license it will make my insurance very expensive :(