thought I would chime in here, it is a totally weird behaviour but it also does happen on sky and it also used to happen for me on plusnet. To make it even more weird I found it disappears when I have codel QoS active on my router. I am not a current aaisp customer, but posted this to assist in diagnostics.

Interestingly it's not as pronounced on your IPv6.

a thoery that all isps get it because we all been routed through GCHQ who have latency spikes on some tasks