Has to be through the ISP.
Have you read
this page?
That gives your next steps if you can't get any sense out of them. At the very least, quite apart from their lack of understanding of the technology, (a) you are not on a 1Mbps connection as you are getting more than that, and (b) if they won't back away from that silly insistence then you have clearly and provably been mis-sold an inferior service.
A referral to Ombudsman Services costs them a few hundred pounds, win or lose

.
To be honest I don't think you have ever reached any technical level in their support staff. Either that, or they don't have any. The whole thing is blindingly obvious.
Somewhere within the last few days I saw a post with a link to the BT Wholesale tool that ISPs use to reset a stuck profile, but I spent a while searching and didn't find it. I may have another go later, but have to go out now.
Good luck.
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site -
www.robertos.me.uk. Domains, site and mail hosting -
Tsohost.
Connection - AAISP Home::1 80/20. Sync 57970/13958kbps @ 600m. -
BQM