If BT Retail is your ISP, then they are the people to complain to, assuming you are not a business customer. BT Retail only deal with residential accounts.
welcome to the BT community forum where customers help customers and only BT employees are the forum mods
please can you post the stats from your router (if hub enter 192.168.1.254 in your browser) and if HH5 then go to troubleshooting then helpdesk and if HH6/SH2 then advanced settings then technical log information .
Have you tried the quiet line test? - dial 17070 option 2 - should hear nothing - best done with a corded phone. if cordless phone you may hear a 'dull hum' which is normal
enter your phone number and post results remember to delete number https://www.broadbandchecker.btwholesale.com/#/ADSL
Is the download/throughput speed ok during the day and poor at peak times? are you using ethernet not wifi?
Someone may then be able to offer help/assistance/suggestions to your problem
Please find below stats.
Further updates,