How can I check how fast my broadband speed will be at an address I am yet to move to?

Have you tried BTs speed checker. You can run it for lines that are not your own. If the house is already on BT broadband and you put in the phone number then they tell you what it gets now so you can not complain you are not getting what they predict. Thus it is best to use the post code option, not the phone number (unless BT changed the way it works)