This is my TBB chart for yesterday on Plusnet FTTC. I've been messing with my network today so there are some outages. The big yellow spikes will probably be the SamKnows speed tests. In the main jitter is negligible.
With cable congestion varies from area to area; with FTTC there isn't really any congestion yet sfaik but if/when it does come along it will be by ISP rather than area. IMO that makes it a lot less likely that there will be significant congestion as ISPs offering an inferior service will risk losing customers.
This is my SamKnows average daily download from when I got their box to now. As it happens the VM congestion in my area seems to have been fixed around the time I gave up on them and ordered FTTC on 3 December. I think you'll spot both that and my move on 19 December too which was predicted at 66Mbps but came in at around 70Mbps. Upstream for me is 17Mbps.
The red line along the bottom is because SamKnows have incorrectly classified me as ADSL2+ upto 20Mbps.
I chose PN over BT as their support is UK based. Unfortunately it's also badlt overloaded right now but they are setting up a new call centre in Sheffield.