It's OpenReach's network, owned by BT Group. Originally a state monopoly. Any qualifying telecom company can access it, provided they meet the criteria. That includes BT Retail, Sky, Virgin Media, IDnet telecom, TalkTalk and many others. All of these, if they meet the criteria, can offer many Wholesale products offered by OpenReach including ADSL, LLU ADSL, FTTC and telephony.
Ask OFCOM and the Competition Commision. Sky don't have to supply all their content to anyone. They chose to supply some of it because it makes commercial sense to sell it via multiple sources. They don't currently offer Sky Atlantic but in the future they may well do.
Virgin Media doesn't have a monopoly on anything.
Here's an article about Sky Sports appearing on BT Vision. Sky have the monopoly on some sport so they are prevented from
only selling it via their own delivery system (i.e. Sky Digital Satellite TV) because if they weren't they could use their sports acquistions as a hook to get more customers to sign up for their delivery system. OFCOM consider it sensible for there to be multiple viable delivery systems of media in the UK.
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Sky isn't forced to
sell (i.e. not give) all of its content to anyone it doesn't wish to sell it to. Sky Atlantic is an example. However Sky doesn't
own some content (e.g. Premiership Football). It simply has the rights to air it and the conditions of anyone owning those rights is that it has to sell it to delivery platforms, including ones it doesn't own, at a fair price. It doesn't have to sell its movie channels to Virgin Media but it choses to do so because it make commercial sense.