Your ISP’s DNS servers (primary and secondary) are likely problematic and waiting for your ISP (Sky) to correct the issues might take ages. Just use something else. You can switch DNS back to Sky any time you like but it's best to use another resolver.
I get the same problem occasionally. The answer is to change the DNS servers you are using.
Just swop between ISP provided DNS servers and Google Public DNS servers(easy to remember).
In my case changing it in the Modem/Router setup pages.
You can also change it in your Web Browser(s) and Windows, devices, but that is too long winded.
That has always resolved the problem.
The root of the problem is the DNS servers.
You can do speed tests on various DNS servers, but I found there was so little difference it was just a waste of time other than a bit of extra knowledge.
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