Pingtest.net is utterly useless. You get wildly different results from different servers that are totally outside any form of control. And then we get all sorts of wild deduction made based on the A-F grading and "jitter" numbers that simply can't be nailed to a cause.
IMO, the most informative state of the network report is PATHPING
WWW.BBC.CO.UK which tells us what's going on at each hop. This needs to be correlated with modem stats and event log and a speed test (preferably not speedtest.net which is nearly as dodgy as pingtest.net.
If you do a series of pathpings, you'll see if there is congestion at the UBR.