In case no one has mentioned it, this is a customer-to-customer help forum. You are not talking directly to BT. With the exception of the moderators, we are all just fellow customers, like yourself.
I’m told the moderators don’t even have account access anymore, so you will need to phone customer support to get this addressed.
A major problem here is the methodology. Most speed test sites only tell you the latency when the connection is 100% saturated. When the upload isn't saturated, the latency can be significantly lower!
It's a problem that can be amplified by the size of the routers data cache. So a good sized cache for a router with a gigabit connection can make a router on xDSL look like it has latency problems.
You might want to see what the results look like on a site such as testmy.net *This is a suggestion, not a recommendation or endorsement*
Believe me I have done every test there is including test my net latency spikes hard there repeatedly too despite the test being light, thats on every server including London and Google (closest available).
I know when desync occurs, I know that constant upload latency alone is currently making my user experience worse when the service is mostly used for an environment where constant download and upload at low ms is expected aka online gaming, voip, 200+ ms is an eternity.
I have friends that can jump on their phones the other side of a house and get 30ms upload.
I did not have these issues with BT at my previous address, clearly im not the only person getting railroaded with high upload latency as most people barely require decent upload speeds / latency and this seems to be a BT thing.
That is on a factory reset router, absolutely zero difference.
Did you read message 11. You should do as suggested as nobody on this forum can help you.