Although BT Business Broadband is a contended service, it’s no longer contended at the exchange level by the previous 20:1 and 50:1 ratios. However, this doesn't mean that you'll receive a one-to-one contention. The contention now happens at the national level and only at busy periods.
BT plans the whole network to provide good performance and shares its national infrastructure between all users. This works well, as consumer use in the business day is low, which gives a very good performance for business customers. An analogy would be the roads near your house. There’s enough capacity to keep traffic moving well at most times and sometimes there’s a bit of a slow-down. If everyone used their car at the same time, there would not be enough capacity.