I don't like aftermarket HID's either and it is about time they did something to regulate their use, but there is a straightforward reason why people do not get fined for using them.
For something to become 'illegal' there has to be an act of parliament or a legal precedent set - neither of which has happened yet with respect to aftermarket HID's. Therefore, they are not illegal.
If you read the Dft fact sheet carefully, you'll note that nowhere does it actually state that aftermarket HID's are illegal, only that it is "the Departments view" that they are illegal and "Under the Road Traffic Act 1988 it is an offence to supply, fit or use vehicle parts which are not legal".
All the talk about ECE regulations only applies to type approval of new vehicles for sale in the UK/Europe, so doesn't apply to used/privately owned vehicles. It also says that HID's are not even mentioned in the UK lighting regulations, so by definition they are not regulated. Until they are, they can't really tell you what you can and can't do with them.
The only part which does suggest you shouldn't use them is "In summary it is not permitted to convert an existing halogen headlamp unit for use with HID bulbs." However this again represents a view, rather than a law or legal precedent.
As for the MOT, it's not concerned with legality but with vehicle condition and safety, so does not consider things like the bulb design but looks only at the resultant function, i.e. beam pattern. Provided the beam pattern is okay and you have two of them mounted symmetrically, you could have nuclear reactors for light sources and you'd get a pass.