I see two possibilities. First your MSIL is reaching capacity due to unexpected demand caused by high 80/20 takeup. Or second, the same effect as it somehow gets throttled to 40Mbps as it approaches 100% of contracted. Then lower if necessary, as demand is rising rapidly these days. Queuing effects will also be present, so even at a 40Mbps MSIL cap you won't get anywhere near that. Motorway slowdown syndrome.
Well it's either that (an intentional throttle/speed cap - seems to be variable, Entanet style - as capacity is reached) or the bad-boy throttle & FUP is still in place (and BT 'forgot' to e-mail me last month saying I'd used too much), but I don't recall using anything like the old 300GB FUP trigger level, that's a heck of a lot of pirated movies, iPlayer and radio streaming (sorry..."Linux distro downloads").
In any case; we're back
to being sub-ADSL2+ speeds again (certainly on download, anyway).
TBB speedtests are becoming a pain (really long time to get a result, mainly because at the very start of the test I get a short high speed burst, over 64Mbps, prompting the TBB test to try a massive file. Then the data speed drops to well below 20Mbps for the remainder of the test - meaning it takes an age to complete).
Still; waiting long enough does produce a result. For the first time (in I don't know how long) the TBB download test agrees with a Speedtest.net result (upload is still way down, but that's another on-going issue which seems completely unrelated to BT's throttling).
TBBmeter FTP upload test reports 15944.5kbps (so 16Mbps, as near as makes no difference), so the low upload result is purely an issue with the TBB speedtest.
vDSL2 FTTC Infinity with BT
DL Sync 80Mbps
UL Sync 20Mbps