MOT regs here -- http://www.motinfo.gov.uk/htdocs/index.htm
As posted earlier in this thread (and in other threads), HIDs are not a fail. The MOT regs say only that washers and self-levelling must work if fitted.
However... what passes an MOT and what is, strictly speaking, legal under the Construction & Use laws are two completely different things. For example, the MOT just says that you need a rear fog light, but the C&U rules say it must have an approval mark, and be 100mm centre-to-centre from any other light, and within a certain height range, and must only be used as a fog lamp and not as something else as well (such as a brake light), etc.
Mr Plod was wrong to say that HIDs will fail the MOT, but, possibly, correct to say that you could be fined for using them without self-levelling and washers.
Next time, give him the URL and tell him to go off and read the rules.