If they are, what's the point of 40/20 anyhow?
As eric has already mentioned it depends entirely on the frequency bands.
The advantage of an 80/20 product being capped at 40/20... is that because the upstream tones are in the lower frequencies.. it may be possible that some lines whilst not able to take advantage of tones in the D3 band, they may well be able to make use of some of the tones in U2 to give more upload speed.
The U2 band isnt heavily shaped by PSD masks - unlike U0 and U1 to a certain extent. So although 40/20 isnt an official wholesale product, Plusnet offering this could actually open up more upstream speeds for longer lines rather than being on the fixed 40/10 product.... even if the line is too long to make use of tones >2750
The band plans are laid down in the dsl standard annexes.. so if someone can make use of U2 even if the line is reasonably long... then why not. I cant actually see any downside to this.