Hi, I might be able to help a little, as I went through something very similar about six months ago.
Sky started calling me to say I was eligible for an upgrade, but they never seemed to ring at a convenient time. I was usually at work, and whenever I asked them to call back later, they never did.
Eventually, I received a letter saying I was eligible for a free upgrade and needed to use the link provided to book an engineer visit. Like you, I clicked the link, but it didn't work.
When I called Sky to ask why I couldn't book the appointment, they told me I wasn't actually eligible for the upgrade. I explained that I wasn't requesting it myself, I was only trying to arrange it because Sky had told me my internet service could be affected if I didn't. Despite that, they still insisted I wasn't eligible, so I gave up and forgot about it.
Then, about a month ago, I received another letter. This time I called Sky first to check it was genuine, and they confirmed that I was due an upgrade. I mentioned that I'd tried to book it previously and had been told I wasn't eligible. They explained that the reason the booking link hadn't worked was because the upgrade wasn't actually available in my area when they sent the original letter, which seems a bit ridiculous!
So my advice would be to be patient. If you can't book it now, leave it for a few weeks and try again. It may simply be that the upgrade hasn't been rolled out in your area yet, despite the letter suggesting otherwise.
