If you follow the Freeview/Freesat route it looks as if you will be limited to the BBC part time HD service and the soon to be launched ITV1 HD . C4HD and FIVE HD are only available if you have a Sky Viewing Card.
It seems as if the BBC and ITV are trying to equalise their HD services on Freeviw and Freesat by degrading the bit rate on satellite so it's no better picture quality than HD will be on terrestrial.
You really need to see some full quality HD from Sky to make a judgement on the various picture quality issues of broadcast HD. If you can also look at some Blu-ray HD on the same receiver.
Good SD is quite acceptable on a medium sized HD TV (say 37" or smaller) but HD does begin to show it's improvement on bigger TV's (say 42" or bigger). However it's all down to personal evaluation of the actual picture quality. Remember some SD is much better than other SD and expect differences.
ITV/BBC broadcast to a horizontal resolution of 1440 whereas Sky broadcast it's own HD channels to a horizontal resolution of 1920 so you do get a better PQ on Sky although BBC/ITV HD can be quite acceptable.
I have Sky HD and do view HD programming when available. However the difference between SD and HD often isn't massive especially when the broadcasters simply upscale SD content to HD resolution.
Is HD worth the £'s you need to spend? Only you can tell but if you are interested in Sport then HD is almost mandatory.