When you signed up you should have been given a personalised estimate of line speed. It is that which is important rather than what the package says. I assume from the 55-78 that this is BT Infinity 2 - you could try and get BT to put you on the lower package.
It depends on who you talk to when you make the order.
When I ordered Infinity 4 end of last year there was no mention of the minimal speed at all and defiantly no mention of an up to speed, or what the speed will go down to in the evening.
A few times now I have lost up to 86% of my speed in the evening, its not too much of an issue.
So I am just saying not all sales team mention the speeds, maybe I was just a one off.