That certainly looks like a bridged tap.
With the first dip at around tone 100-110, one formula tells you the extra wire is about 340ft in length.
Another method, when looking at the dips at tone 100 and 1,000 (with 3 sub-dips inbetween), suggests a length of roughly 100m.
Could that relate to something in the house or premises? or the length of the drop wire?
Persuading BT, though, isn't so easy.
The usual way would be through the consequences the bridged tap causes - ongoing SNR variation and disconnection problems, or significantly low speed.
Unfortunately, the downstream doesn't seem to help you there. While you get troughs in the Hlog (and signal), you also get troughs in the QLN ... so the bitloading stays relatively even ... so the sync speed isn't particularly affected.
Upstream is different, perhaps affected by the way UPBO works to adjust upstream power. There are dips in Hlog, but no difference in QLN, so there are significant dips in the bitloading. Upstream is slower than it would otherwise be ... but I don't think Openreach have fault threshold in this direction. And, as it reaches 10Mbps anyway, it might be hard to persuade anyone of a fault worth pursuing.