Sky Fibre runs on the BT openreach infrastructure, ie the same as if you had a BT internet connection. In most cases this is Fibre to the cabinet (FTTC) and then goes down the copper line between your house and the local cabinet. Virgin fibre is
mostly fibre to the house (FTTH) although they do resell the openreach infra just the same as Sky do. FTTH is generally faster than FTTC unles there is a technical problem with the line.
I have Virgin FTTH and notice every now and again the line needs resetting (about once a year) but the WiFi from the virgin supplied router is absolutely fine.
Thus I would suggest that maybe there was a technical issue with the Virgin line which was undetected, or that there may have been other things affecting your WiFi signal such as noise or conflicting WiFI channels being used by your neighbours. A channel scan with your smartphone could help you identify a part of the spectrum which is available

I was with Sky for many years and they were perfectly fine, my only beef came when I moved house and cancelled my connection for a certain date (ie just after move out) but they cut it off 2 weeks prior and when you work from home that meant I had to be connected to the internet all the time thus I had to use my mobile phone as a hotspot. Cue a mobile bill of over £400 that I had to suck up.