Brians pretty much got it on, not sure on the not dazzle road users, if they come on with hi-beam, your gonna dazzle em anyway, lights should be dipped if passing other vehicles.
The thing is, you can put on whatever you want, and it'll pass an MOT, auxillery lights are not tested, and will only fail if they have sharp edges etc, which is why many get away with non legal lights, on previous motors I've got through MOTs with 100W headlight bulbs (before I knew any better).
Personally, I have a light bar, wired so they can be operated with the hi-beam, or independently, or not at all.