I think you will find that BT do indead have a minimum garanteed speed, and I also got it confirmed in an email. I was getting much more than the minimum garanteed speed but now I am having problems with it and it's now below it. I just had an engineer out to fix it, but they didn't and said the speed I'm getting is fast enough and and they also said that I should not have given a minimum speed garanteed by anyone.
I have moved your post to this new thread, so you can get personal help from the community or moderators, so please watch this thread for updates.
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Which BT Home hub do you have?