Unbelievable high ping across whole network

We are with EE broadband (this issue started when we went with them a year ago), do NOT have full fibre and are based in the UK.

For around a year now, a common occurrence happens in our WHOLE network where the ping will skyrocket to a point where the internet may as well be turned off because it takes so long to load up websites, play games, work etc. When I say "skyrocket" I mean the ping goes up to 1300ms+ which can go on for hours and I think it mostly happens when someone is downloading something or when there are multiple people on devices BUT it does happen without these factors anyway.

I'm pretty sure the issue is within our local network because the spikes in latency happen on both the default gateway and our IP and other IP's (e.g 8.8.8.8) at the same time when using cmd. I think this means it's in our local network, but I'm not an expert on home networking hence why I'm here so that may need confirming.

We have had engineers out, replaced routers and it's all still the same. Maybe it's the copper wire outside under the ground? Maybe it's faulty cables? We use ethernet cables going straight to the router. I'm normally quite lucky when it comes to fixing network issues and somehow figure out a way to solve problems but after a year with this problem... I'm completely stumped and fed up.

Any advice would be appreciated, again I'm not at all an expert so may have gotten things completely wrong in this post so please go easy on me 😅