The BT Wholesale Carrier Price List here gives prices that it charges other telcos to terminate calls to its NGNs or transit to other terminating providers.

See section B1.06:

Quote:

If I've understood it correctly, then BT makes a loss on these numbers during the evening and weekend. Remember that the retail prices must be compared without VAT, and not with as we commonly think of them. The loss shows how much they make from them during the weekday daytime.