Will BT Internet or Sky let me cancel a new fibre-optic contract if the speeds are as low at 3mbps - 5mbps?

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I am considering upgrading to a fibre-optic connection. I just need to know if BT Internet or Sky will compensate in one way or another if the internet speeds are much lower than promised (for e.g. if speeds are slow as 3mbps-5mbps - which is the speed I get from my current Sky broadband connection.

Any guidance would be appreciated. If you have dealt with either of the aforementioned companies in the past please advise what would be the best course of action other than downgrading.

Thank you.
What about this:

http://crave.cnet.co.uk/gadgets/bt-infin...-50010340/