Yeah the tech support agents aren't always knowledgeable on these things. Have you checked your Max Traffic Rate for the Upstream on the modem admin pages? Mine is currently 38500089 which roughly equates to 35-36Mb after overheads. It could then be congestion that is causing the lower upload but 24 is still better than most.
I wouldn't be able to go back to lower than 20 up after being on the BT FTTP 80/20 package.
No congestion in my area because as both down/up speeds are same even at peak time. The upload config is already updated, but look like the max upload I can get is 1/3 less than max speed, because my upload speed was 15Mb out of the max 21Mb before the speed increase.
The 24Mb I am getting is not bad at all and within the guideline for the M350 package which is 21Mb but I want to access files from my home NAS sometimes which is why I went for M350 package for better upload.