Could not help notice that every single one of those links in the auto generated Related Topic section are unresolved.
Check out this thread and its solution.
https://community.bt.com/t5/BT-Fibre-broadband/Horrible-ping-spikes-and-packet-loss/m-p/1915395
It proves BT may be able to help from their end
The more info you share about the things you have already tried to resolve like checking the obvious stuff cables, updated firmware, fully updated operating systems and applications, reset router, checked line noise, show network debugging info like ookla speed test showing jitter and packet loss or use other tools to show the problem causing the higher latency is not on your local network - winmtr, ping, traceroute etc etc might help others to help you
