Update: 4th engineer came out today. I explained the only issue was dl speed (ul speed now uncapped and over 100). He checked it using his own laptop, they always seem to use fast.com to check speeds and it was consistently in the 600s which tbh was as high as I've ever seen it, however, he was adamant something was wrong as it should be consistently 800+.
He checked the line and found no faults. He changed the ONT box, he changed the wiring outside the house from the external box, he tried new ethernet cables, did everything and still... dl speed in the 5-600s. I'm happy at this stage for him to just report his findings back to EE but he isn't ready to give up. He rang a department higher up and went sat in his van, they did some line checks etc. He had to give up in the end as there was an issue with their test servers, he wanted to show a 1 gig speed from their own test servers therefore eliminating themselves as the cause of the slow speed and thus pass it back to EE. Was here for almost an hour and a half and was insisting they need to find out why its not hitting max speeds when its a 1 gig line they've installed. He's coming back tomorrow at 4.30pm to run the speed checks again.
EE arent happy until the line shows full speeds as they are providing this service to customers via BT hence why they keep sending Openreach engineers out to me. BT want to show me a one gig line up and running via their test servers to demonstrate the line itself is fine and pass it back to EE, hence the back and forth.
Doesn't bother me tbh as the line itself is solid, pings are good, its more than fast enough for me. I'll leave EE and BT going at it.