I understand that I'm not guaranteed 900 but I've never reached over 800 and rarely get over 600. Is this something I have to swallow or is it something BT could sort? The engineer mentioned something about throttling.
As stated on your other thread, FTTP is a contended service, which is why it is so cheap. The backhaul capacity to the GPON splitter node is 2.5Gb/s (or in some newer installations 10Gb/s) which is shared between up to 32 users. If you want an uncontended service, you would need to pay Megabucks for a leased line.