Is there a particular reason why you want to use BT Infinity? The latter is not the best telecom product, it's not future proof, and in many cases it's not suitable for business needs.
In fact, even though BT doesn't publicly admit it, it is not really interested in the business market. There are loads of business parks, industrial estates, and small towns without adequate telecom services from BT.
Take a look at this forum to see how incompetent BTs sales and support team have become, in our case BTnet hasn't even been able to provide us with a simple quote for requested telecom products.