I've seen this problem before back when I was with O2. It took a long time for the capacity to be upgraded. So long infact I switched to Plus Net on a 3 month contract then switched back to O2 as that gave them enough time to fix the issue.
Sky might be reliant on Openreach to upgraded Sky's Ethernet Backhaul Direct service in your exchange. Or Sky could be waiting on a new service delivered as part of their new contract with Virgin Media. Either way it can take 3-6 months from order for the upgrade to actually happen ... yes Openreach can be that slow!
Part of the slowness I suspect is simply down to Openreach ensuring BT retail has the upper hand on its competitors.
This assumes it's a problem with Sky's capacity in the exchange. it could be the GEA cable link (also in the exchange between Sky's rack and backhaul to Openreach's fibre switch that connects all cabinets served by the exchange). Or it might be congestion on the fibre to your individual cabinet. If either of these reasons then ADSL2+ services won't be affected.
Really you need to see ping graphs for a mix of ISP's and cabinets served by the same cabinet as you.
It's far easier for people to set-up a think broadband graph and share this.
See mine for my Sky Fibre at Canterbury in my signature below. The spikes in ping are my SamKnows box testing the line.