Probably victim to a fox, cat or disease.
Which cygnets do you mean? I've been following the family on the New River, which has 7 cygnets. They still do have 7 but unfortunately one seems to have been rejected, and is now spending most of its time on its own. I believe Woodberry Wetlands are aware and keeping an eye on it, but do hope the poor little thing survives. :(
The other six are now big and noisy and still hanging out with Mum & dad.
Sounds like different families, yes. It's unusual for them to leave their parents much before the spring of the following year, so I hope they haven't come a cropper. I also wonder if perhaps the extreme hot weather is somehow affecting their usual behaviour? Would need a swan expert to confirm, but it does seem odd that two reports of dwindling swan families came in the same week.
It sounds like you saw the little dude I have been watching :(. It's smaller than the others and perhaps seen as a 'runt'. It did seem to be getting on with business as usual and feeding itself on the few times I saw it alone, and others have apparently seen all 7 of them preening together in the early morning. But I've mainly seen it on its own this past week.
London Programme on BBC Monday night 6/8/18 discussed issue that due to hot still / Low water. That the Green slug gets Hot and gives off botulism , which will poison the cygnets. and Kill them in a Short Period
Many years ago I used to Work at Finsbury Park. In those day's the Cricket Pitch was a Secure / Dog Free Area
The Swans used to nest in the weeds, in that area.
The New River has many Large Pike that used to eat Birds that landed on the River
Also remember finding many Swans attacked by Large Wild Cats. Dogs seem more afraid of Flapping Wings
Whether good planning or good luck - a nice capture.