I had my MoT test with my HIDs in. HIDs are fine for UK MoT, it's just the beam pattern/aim that is tested, so mine obviously tested ok.
However, passing the MoT doesn't prevent the police from stopping you and seeing if your car is roadworthy if they suspect otherwise. In *theory* aftermarket HIDs like the ones typically retrofitted to the IS200 are illegal, but I don't think this has been tested in practice. As long as you haven't got very blue/purple HIDs (e.g. 10,000K) then I think you'll be ok if the police stop you.
A similar example is if you've removed all your cats. You won't be able to pass the emissions section of the MoT test like this, so some people will insert a cat just for the MoT, then take it out again. However, if for some reason you get stopped by the police for a roadside emissions test, and you fail it because you've got no cat, you're going to be in trouble - but you'd be in trouble for failing this test, it effectively doesn't really matter that you've swapped your cars setup from the time it was MoT'd.