Right, from experience I have ran them in:
1. Projector Headlamps ... 8000k on my previous car - Never failed an MOT, never had problem from the Police.
2. Ran 8000k bluer bulbs in a bog standard pair of headlamps (my Golf infact) and never had any problems in MOT or Police, I have since sold them and now back to sh!tty yellow bulbs just because I got bored off the 'extra' attention it brings and the fact I finally realised it doesn't suit my car as ive kept most things standard on it.
So the verdict being. Yes they are illegal by the book, however police don't care unless they just feel like doing you for having them one day, most MOT testers don't care, but again depending on where you go, you could fail.
They are ONLY LEGAL if you have headlight washers, and the lamps are projector lenses and all aligned correctly etc.
EDIT: also, they cost around £40-£50 on ebay and easily removeable ... so worst comes to worst, run them when you feel like, like a seasonal thing, its a 5 min job to take them out before an MOT aswell ... you know how it is.