The difference it that it would either connect at 1Mbps, or nor at all. So if it sometimes connects at 800Mbps, on a fixed 1Mbps line it wouldn't.
That also sounds like speed test or download speeds you are quoting, not connection speeds.
IIRC the actual connection speed of a 1Mbps line is 1172kbps, which I think would give you a maximum download speed of about 985kbps.
Your broadband is in fact provided by TalkTalk, so probably suffers from the inadequate provision of all non-LLU services, even though I expect it will be TT business Wholesale not TalkTalk Retail. The TalkTalk Retail product is no longer sold, (I expect there are legacy customers), but had a very bad reputation just like Sky Connect.
Are you able to get the line stats from your router, particularly when speeds are bad? What I'm saying above is that you are probably suffering from congestion, not a low connection speed, unless you have already established it is connection speed.
Have you done the stuff we usually recommend here, such as removing the ring wire
and preferably also extension cabling? Particularly plug-in extension cables
My broadband basic info/help site - www.robertos.me.uk
| Domains,site and mail hosting - Tsohost
Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 56.4/14.5Mbps @ 600m. - IPv4BQM
"Angels can fly because they can take themselves lightly."
- G K Chesterton.