To update people joining the thread which is getting quite long. Sky have acknowledged there is an issue this post. It seems to mainly affect Three's customers but at least one other mobile operator could be affected but I was not given a name.
The cause is most likely to be a change in how Three's network is routing requests to Sky's servers. The fact that VPNs avoid the issue supports that theory. However using a VPN is a work round not a solution.
This is not the first time I have seen issues between Sky's systems and specific ISPs network set ups and in the past we have been told that there are technical liaison channels between companies where this type of thing is sorted. Customer Service reps at Sky or Three will not be briefed on what is going on so you may get reps personal theories but this thread is likely to be the best source of serious info.
The speed Sky replied to my query in the closed Superuser forum shows it is being taken seriously by the right people
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65inch Sky Glass, 3 Sky Streaming Pucks, Sky Ultrafast + and Sky SR213(white Wifi Max hub) main Wifi from 3 TP-Link Deco M4 units in access point mode
Just getting “ please waiting for your program to load”
