I purchased an upgrade from my existing 300/50 service, and the order completed yesterday.
My upload speed is now a solid ~100mbps, but my download has not changed - still tops out at around 300mbps.
I have tested this from a linux box connected over ethernet with Cat6e cables. I can't use the BT speed tester any more (seems to be a known Smart Hub 2 issue), but fast.com and a wget download of a 2.6GB linux image both top out at around 300mbps.
I phoned for support this morning. They told me the actual fibre connection is connected at around 810mbps (that's something I may follow up separately) and blamed the computer doing the testing (antivirus or other software or something, which isn't the case - it can saturate its gigabit ethernet on local network transfer).
Any ideas for where I can go next? At present I'm paying a premium for a service I'm not receiving.