You're a little stuck if you want to grab BBC, ITV and C4 in HD. ITV-HD is only available via Freesat as a red-button service. It's not on Sky. And 4HD is not on Freesat, it's only on Sky.
ITV-HD is set up in this way to prevent it being available to any receiver that isn't Freesat approved (i.e. so Sky can't have it). The signal isn't encrypted as such (because Freesat is supposed to be free, without encryption or conditional access cards required) but delivering it as part of Freesat's interactive service effectively prevents it from being received by any standard free-to-air satellite receiver, or a Sky receiver.
The eye-tv solution ShadowTD mentioned will not work with Sky - its CAM slot is not compatible with Sky subscriber cards (because they use a non-standard system), and it won't work with ITV-HD because it can't receive Freesat's interactive components.
If you want to receive all the PSB channels in HD, then you will need a Freesat-HD box (for BBC-HD and ITV-HD), a Sky-HD box (for BBC-HD and 4HD).
You then need some means of recording your programmes ... and there I bow out of the discussion, as I haven't a clue about that side of things. It must be possible because HD recordings of UK TV shows turn up all the time on the various useful-but-legally-dubious download services all over the internet.
... Basically, the eye-tv will work if you only want BBC-HD (although it is inevitable that at some point in the future, ITV and C4 will also put their HD channels on a free-to-air footing). The simplest, and cheapest, solution would be for you to acquire HD content via the internet and take screen caps while playing it on your Mac through the VNC Player (
downloadable from here).