Stick your post Code into the Wolfbane predictor...
http://www.wolfbane.com/cgi-bin/tvd.exe?
and see what it recommends as the aerial required. It's not guaranteed to be 100% accurate but if it comes up with something like High Gain Amplified or similar then you may struggle to get anything on a set top aeria.
The problem with such aerials is that they are lower gain than a roof top or loft aerial and in general mounted low down and indoors. These two factors conspire to reduce the amount of signal you get out of the aerial. The more walls between the aerial and transmitter the worse it will be.
And it is lack of signal that is the killer with digital, not helped by the low transmitted powers to start with.
If you have a loft space then why not fit an aerial in there? Fitting a roof aerial is the best option and to be honest your chances of having the telly fried by Lightening are pretty remote (though not as remote as winning the Lottery apparently

) And of course there is nothing in the rule book that says an aerial HAS to be the highest point on the building either. If circumstances allow then you could mount the aerial to a wall below roof level. Like that it would be as vunerable as your Sky dish.......