You're confusing Virgin+/Sky+ with Sky HD: the two are totally unrelated. The first is a box that allows you to record to it, like a PVR. The box (like any cable/sky box) does also decrypt and process the signals coming to it from the satellite/cable, and yes for the Sky+ etc it does contain a HD tuner (so it would be easy to assume the two were the same thing), but the two processes are completely separate and should not be looked at as being one.
Once the digital switchover is complete (i.e. they've switched off all the analogue transmissions), if you don't have a sat/cable connection you will not be able to watch any TV without a freeview decoder. Whether you want to buy a £20 box, or whether you want it built in to your TV, is up to you.
Freeview HD is potentially a feature really worth having on a telly, again particularly if you don't have a cable/sat connection (or do, but don't want to pay for a HD box/subscription, or do, but only in one room for example). It means you can just plug a normal roof aerial in the back and it'll not only decode all the freeview stuff (meaning you can watch TV without a separate box), but it'll decode all the HD stuff too (all the World Cup games for example are broadcast in HD on the regular airwaves, through BBC and ITV HD channels). The prevalence of HD on freeview is only going to rise, and it's also never going to cost anything to view. Of course the alternative is to buy a HD freeview box as and when you need it- they are only going to come down in price (£100 is abuot the cheapest at the moment).