Hi, I think you need to think about why you want the trees 12 foot high (and unless you prune them twice a year they will soon be a lot higher!). If it's to provide screening between you and a neighbouring property, and espcially if you are very near that property or to the South or West, then your neighbours may well have a case to force you to take them down. The other respondents are right, there is no law as such that says you can't have a hedge more than 6 foot high, but your neighbours can take action aganist you if they feel that you have planted a hedge which adversely affects them. It's not just conifers - it's any trees which are planted as a hedge.
One of the problems with conifers is that very little will grow near them, so there is a knock-on impact on the rest of your garden. Plus they are incredibly ugly. We have a leylandii hedge at the front of our property, dividing our front garden from the neighbours, which the neighbours planted. they keep it to a reasonable height, and it doesn't impinge on our light, so it's not a problem from that point of view. But nothing grows under it so it's very boring to look at from our front room.
If you want screening, have you thought about using a mixed hedge (yew, hawthorn, beech etc) which would be much more attractive. If you waited until autumn you could buy rootball plants which would be much cheaper!