Don't forget that your 14 day right to cancel begins the day after you first agree a contract and not 14 days from the beginning of your service. This could mean you being liable for charges due to early cancellation.
You are better letting Plusnet an opportunity to investigate your poor speeds and if they are not able to reach the minimum guaranteed speed you would be able to leave without penalty.
Ive had the exact same problem. There is no one to contact as per my post earlier.
I did notice that the 14 days cooling off cancellation does start from the date of order which i feel very clever marketing all be it very wrong that they can get away with it. They give you your activation date outside the 14 days of your order.
absolutely disgusting company