I'm a UK resident who has built a number of Media Centers.
In the UK we have :
Analogue PAL I OTA with NICAM digital stereo sound and World Systems Teletext - being phased out region by region, due to be completely switched off during 2012. This is 4:3 only and probably best ignored.
"Freeview" Digital DVB-T OTA - used to currently carry 6 multiplexes of mainly FTA (free-to-air - i.e. non-pay TV, non-encrypted) services. This is a popular service, and has a 7 day OTA EPG broadcast on all channels, as well as Freeview+ PVR support (for series link metadata, broadcaster signalled record start/stop, overrun protection etc.) It also carries MHEG5 interactive digital text. All Freeview SD content is MPEG2. There are a couple of encrypted Pay-TV channels on this platform - but not many. Most broadcasts are 16:9 SD widescreen.
"Freeview HD" Digital DVB-T2 OTA. Launching later this year, Freeview HD will see one of the six DVB-T muxes converted to DVB-T2 (a new modulation system) allowing 36Mbs (compared to 18-24Mbs with DVB-T) to be carried in a single channel. BBC HD, ITV HD, C4 HD and possibly a.n.other channel should be launching on this platform, using H264 for HD broadcasts. (No announcement if 720/50p or 1080/50i - and ITV have started questioning if they can afford it). Currently no PC or set top boxes on the market support this standard. The UK will be the first country to use DVB-T2. Test broadcasts are underway though.
"Freesat" Digital DVB-S Satellite on Astra 2 28.2/Eurobird 28.5 - carrying a few more channels, and a slightly different line-up, than Freeview, and uses a proprietary EPG, and MHEG5 interactive digital text. Some HD services are carried on Freesat (BBC HD, ITV HD, Luxe TV HD) - and they use H264 for 1080/50i HD broadcast (using DVB-S). They are all currently DVB-S - though all Freesat HD receivers have to be DVB-S2 compatible as the Freesat HD broadcasts may switch to DVB-S2 in the future. There is no encryption on the Freesat platform.
Sky SD Digital DVB-S Satellite on Astra 2 28.2/Eurobird 28.5 - the main UK Pay-TV platform, SD MPEG2. This uses Videoguard/NDS encryption, a different proprietary EPG, and OpenTV middleware for digital text and interactive TV. Official Conditional Access Modules (that allow third party receivers and PC tuner cards to decrypt with a valid viewing card) for the NDS/Videoguard encryption system are not available - though there are some reverse-engineered CAMs that will allow you to do this with a suitable DVB-S receiver system with a CI (Common Interface) slot - such as the Digital Everywhere Firewire receivers. However you still need an official Sky receiver to do this, as the card needs to be authorised (which means putting it back into a receiver once a week overnight) and married (which requires a unique valid receiver serial number for the card to be married to) Most major channels are 16:9 SD, though there are a lot of cheaper 4:3 SD channels out there.
Sky HD Digital DVB-S2. As above but using DVB-S2 and H264 1080/50i for most of the HD channels.
Virgin Media Cable. DVB-C, not sure of the encryption scheme. Mainly SD with some MPEG2 HD. Not as popular as Sky SD/HD.
NB : A couple of channels that are non-subscription, non-Pay TV, are still only available on the Sky platform, and not-Freesat, as they are encrypted using VideoGuard/NDS as "Free-to-View" and require a viewing card. Channel Four HD is the most obvious example - it is not available on Freesat HD, only on Sky HD. At one point all the main channels were FTV only - but slowly they have moved to Astra 2D (which is a tight beam on the UK) and have been able to ditch the encryption and go FTA - allowing Freesat to launch.
For a Media Center you have a choice of a couple of options :
DVB-T Freeview. Loads of choices - and quite a lot of dual-tuner cards. The Nova-T 500 PCI card is quite popular in XP MCE and Vista MC installations, though has a few issues with Win 7. There are lots of options though - including dual-tuner USB, single-tuner USB, PCI card, Firewire etc. Vista TV Pack / Win 7 added proper support for DVB subtitles (we don't used closed captions in the UK for broadcast) and MHEG5 "Press Red" interactive digital text. (There are two full-screen and 4 quarter screen interactive video services on Freeview)
DVB-S Freesat (only officially supported in Vista TV Pack and Win 7). Quite a few choices - the Nova-S2 HD (which is DVB-S2 compatible) is popular with Vista TV Pack / Win 7 owners. Only Win 7 officially supports H264 HD broadcasts though TV Pack can be hacked to add it. This will also support DVB-S2 if Freesat HD broadcasters switch from S to S2. Other Firewire, USB and PCI/PCI-E cards are available. MHEG5 support isn't currently enabled in Win 7 / TV Pack for DVB-S - but it can be persuaded to work with a registry hack (and allows ITV HD to work) - though it is buggy (hence it not being enabled I guess!)
DVB-T/S combined. Vista TV Pack and Win 7 both allow multiple tuner cards on different platforms to be deployed. I have a quad PCI-E Pinnacle 7010ix (£30 on eBay - bargain!) tuner card in a Win7 RC box which has 2 x DVB-T and 2 x DVB-S tuners on a single card. You feed it one OTA aerial/antenna feed and 2 LNB feeds from your dish. This will get you all the Freeview SD and all the Freesat SD (and all current Freesat HD - though if they switch to DVB-S2 you won't get them)
DVB-S/S2 Sky. Digital Everywhere FireDTV S2 (external) or Floppy DTV S2 (internal) with a suitable CAM. The SD MPEG2 stuff can be used in XP MCE and Vista - as D.E. have produced drivers that pretend to be DVB-T even though they are actually DVB-S. With other software, or with Win 7 RC (and Vista TV Pack hacked with H264 support) you can also get Sky HD channels. This will get you all the encrypted pay-TV services you subscribe to - using your Sky viewing card.
HOWEVER the reverse engineered CAMs are not guaranteed, and certainly not supported by Sky. They could stop working if Sky tweak their encryption - there is a card swap (the second in the Sky digital platform's 10 years of operation) currently taking place...
The other way of getting Sky SD is to use an analogue capture card, and an IR blaster to control a conventional Sky SD reciever. However this has poor picture quality (S-video at best) and is a bit klunky...
There are lots of other ways of doing TV in the UK - MythTV, SageTV, GBPVR, DVBViewer etc. - however Media Center has a thriving community. Media Portal is also proving popular.www.avforums.com
has a strong HTPC section that is tailored to the UK market.