police wise you'll probably be fine - for example VAG cars with HIDs have pop-up washers that are flush with the bumper and use the water pressure to rise up and squirt (ooh er).
LIn addition, the feds will only really know you've got HID's at night...so they won't see anyway. - unless you get pulled over for something, of course, in which case they'll more than likely pick it up during the whole "is this your car sir?" tyre and bulb checking "lets see what else we can do him for" occurrences.
It's therefore basically down to whether the MOT guys notice / care. I must admit, I don't know the actual reason for such a rule. Potentially something to do with refraction of light on a dirty lens making it dazzling to other road users, but thats speculation on my part.