Didn't BT do a 40/20 product a few years back, being basically the 80/20 service capped at half speed download?
I'm sure Plusnet also offered the same package. It was withdrawn in favour of the 40/10 option some time later.
In that case the OP is probably on that legacy package.
No they didn't, but yes Plusnet did.
However it still wasn't a 40/20 syncing product, no such product has ever existed.
Plusnet provisioned those customers on 80/20 and used an IP profile to cap downstream throughput.
What's probably happened here is...
A couple years ago BT upgraded customers on BT Infinity 1 (which was 55/10) to 80/20 for free.
It was discussed
in this thread.
The OP's line is definitely 80/20 but DLM has capped the downstream to 44Mb.