As for handback threshold, I don't actually know for sky, but I think one can test as I think they have a site that will tell you what they guarantee. Maybe someone with an FTTC line can try.
However, the reason I assumed they use the "handback threshold", and that we currently use it and suggested on our page, is simply that this is the threshold we can get BTW to hand back and refund to us with no minimum term or costs other than our own.
I would be surprised if they go beyond that as (a) it is not really needed for their headline marketing message - the exact threshold chosen does not really matter there, and (b) it could cost them money, especially until the 12 month minimum term is dropped by BTW.
I agree 10th percentile is crazy, and I would love to see BT guarantee the speed they actually have as the lowest in their speed estimate range. The used to, and used to even have a clause saying they would actually try and fix the service if it did not meet it (at least try), but now it is just "take it or leave it" at the (lower) handback threshold.
The reason it is ridiculous, regardless of which threshold applies here, is that they are not actually offering anything better than any other ISP in terms of speed of YOUR line. Your line will get what it gets. They won't actually make it get 55Mb/s, and if the estimate is too low by some criteria (i.e. they don't think it will get 55Mb/s) then they won't even try.
This is very different to what people seeing the headline will think - such people will think "I'll go to Sky as they *guarantee* I'll get 55Mb/s - other ISPs don't guarantee that, they are all slower", or some such.
At the end of the day I can't speak for Sky.