The easiest / cheapest way to see HD is to get a Freesat box.
Freesat is an offering from BBC/ITV but you do need to fit a dish to your wall and buy a Freesat box.
If you have an existing Sky dish you can use that and then you only need to buy the box.
There is no monthly charge.
(Note some Freesat boxes are NOT HD so make sure you buy one that IS HD).
(Note also that Sky have an offering call Freesat, a different thing altogether, but that dones not show HD)
More on the BBC/ITV freesat here
http://www.freesat.co.uk/