You seem to be misinformed.
BT is supplying a service. The cable and the router - if supplied buy BT is part of that service. ( Openreach is only legally a subcontractor). The problem is if the REIN is causing interference the the BT broadband icannot sync properly and Pings > 100 ms, long latency, packets dropped and the apparent broadband speed will be all over the place.( even drop away totally)
This can even happen when Repetitive Electrical Impulse Noise (REIN) and Single Isolated Impulse Noise (SHINE) are not evident.
BT supplied a Netgear router to me because they could not get the Smarthub5 to work because the hub would not sync. The BT exchange equipment is not robust and therefore the service does not meet contract.
Hence pipe but no water.

If the device can be identified and owner of it insists it remains then maybe chasing for a product replacement with the supplier of device quoting CE regulations at them might get somewhere if its fairly new, or if its not expensive just sourcing a replacement yourself may help.

Having worked on REIN issues in the past, I'm far from misinformed.

And there is zero obligation on BT to provide the service, under consumer law so long as they've undertaken reasonable steps to fix the issue (which sounds like they have) then they can put their hands up and say we've tried and give you the choice to walk away without any penalty.

Once the Broadband USO is in place things may be different, but even then they will not be able to seize REIN producing kit, and they will be cost limits involved to bring a premise above 10 Mbps.

Exchange equipment is irrelevant since we are talking Fibre to the Cabinet.

There is a small chance that trying a different modem on the line might help, but no absolute right to request that BT go through the permutations of kit, i.e. MJV you were lucky

This was my thinking.

I've known it happen where Openreach couldn't get in touch with the owner of the problematic kit and thus deemed them non co-opperative.

When the customer having the problem spoke to the neighbour they were quite shocked as rhey had no idea. They were happy to split the cost of replacing it. It was an old "faulty" Sky box.

Sky box,
Florescent lights
Treadmills

The oft cited sources.

Polite chats with the neighbours is the best way forward, Ofcom is unlikely to act for just a single user affected.

Thanks. I've asked all my 3 immediate neighbours. Openreach have not spoken to any of them. It feels like the problem is with one of the houses on the Main Street.

BT verbally accepted to cancel contract. I insisted on them sending an email which they did. But they are refusing to let me go without a termination fee! They say the email is not binding. I've got less than a month to go. Can't wait.

BT are not allowing me to cancel the contract without a penalty! I've tried 3 Smart hubs, 1 home hub 5 and 1 TP link Archer modem router. No use whatsoever.

The strange thing was I was getting uninterrupted internet connection albeit @ 4Mbps. When I called bt they said they've reduced the speed on my line to achieve stability. And that if the speed was increased, I would start having interruptions again for long hours. I told them to leave it as it is. But the connection became unstable with no internet for up to 12 hours.
Another strange observation was, my WhatsApp used to work all the time despite BT hub flashing orange ( this indicates no connection) !