Is this a 1st fix? If so, I wouldn't even think about using 12v in a bedroom, why go to the expense of transformers? If you are worried about cables, you are just increasing the current in them by using 12v. If you have a choice, why would you concider running the cables beneath the insulation? LED v Halogen, you get what you pay for. If you want to pay ÂŁ2 for an LED light, they will be dim. Pay ÂŁ10 - ÂŁ15 they will be just as bright as anything else out there with the added benefit of the long life they give as well as the energy saving benefits.