For the heat argument, it shouldn't be much of a problem - many do it and the heat through a proper chimney will be easily managable by the TV. Most important is making sure you have a proper anchor point to stick to.
For the height, official answer is generally:
Your "eyes" should be about 1/3 of the way up the screen. When sitting in my couch, my eyes are about 37" from the ground. Based on that, you'd want the bottom of the set to be about 20" from the ground.
B=bottom of TV
37 - (50/3)= 20.3
Otherwise as others said you will have it too high for long periods, unless you lay down or slouch alot. If you are intent on it, to really capture the chav look you should have the cables just dangling behind the TV not chased into the wall. And the box for the TV should be left in the garden for about a month