I can break the 300mbps barrier on other speed tests such as dsl reports which has 12 threads. I'm not sure what the normal pattern is for FTTP but the speed builds very gradually and only maxes out at the very end of the speed test.
Your latency of 37ms seems very high for FTTP - and higher latency does cause TCP to ramp up more slowly.
Are you on a direct wired connection to your router when running these tests?
Can you check the port status to see if the NICs are auto-negotiating to 1Gbps - both between PC and router, and between router and ONT? (if your router will show this)
Are you running Windows? Try booting your machine from a Ubuntu Live USB and doing the test with that. I've seen laptops which max out at 150Mbps when running Windows but can fill a full 1Gbps when running Linux.
Alternatively, if you know someone who has a Mac laptop with a gigabit ethernet adapter, ask them to come round and do a wired speedtest.
It depends on where they are, they might be far from London, or the TBB Servers.
I am about 8 or so miles from the London eye and I get the following:
But it says my latency is 21ms yet my BQM says its about 2 to 3 ms between me and TBB server, same when I ping google.com is 4ms and the bbc.co.uk is 2 to 3 ms.
I also get a latency of about 12ms from our dedicated rack server located in France, so not sure what is going on with the latency on the TBB Speed test, I think there has been posts about that topic.
A traceroute to the test server used:
tracert -4 speedtest7.thinkbroadband.com
Tracing route to speedtest7.thinkbroadband.com [80.249.106.133]
over a maximum of 30 hops:
1 <1 ms <1 ms <1 ms Yazoo [192.168.2.253]
2 1 ms 1 ms 1 ms 172.16.10.194
3 * * * Request timed out.
4 3 ms 4 ms 3 ms 31.55.186.180
5 3 ms 3 ms 3 ms 195.99.127.106
6 2 ms 2 ms 2 ms peer8-et-0-1-3.telehouse.ukcore.bt.net [109.159.252.98]
7 2 ms 2 ms 2 ms linx-gw2.thdo.ncuk.net [195.66.236.240]
8 4 ms 3 ms 3 ms po4-31.core-rs2.thdo.ncuk.net [80.249.97.85]
9 2 ms 2 ms 2 ms speedtest7.thinkbroadband.com [80.249.106.133]
Trace complete.
Which matches my BQM, so yeah a strange thing LOL
I am also not that far from Telehouse so that might help as well
Paul