Can we get back to basics for a moment. Why do you want Broadband in the first place? If you are satisfied with your connection speed and existing system do you need to upgrade?
If you change then as I see it you will need BT to provide two BB connections, one for your business and one for your home. Each line will have an ADSL filter on it which means that one half of the line will carry data and the other half voice. One or both of the BB connections can be used by your MACs. You will now have two t/p numbers, one for the office and one for home and you will pay BT for them. You say you already have a Service Provider who can handle BB so they should be able to continue to offer you their services and you will pay them for that. I see no reason why should not also be able to continue with Mercury service over the new BT line.
What I don't see at the moment is any money saving ideas out of that lot, unless I've missed something.
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