That only equates to a pound a minute, which I agree is a lot, but call box calls are generally more than home phone calls anyway, so maybe it's proportionate.
The money goes towards BT's profits.
There's no obligation on this service for BT to charge in proportion to its costs and there is no competition. In addition I suggest that few people actually want to do it these days.
If you really need to make this link-up, have you tried phoning the box from the mobile - it will probably work out cheaper for most mobile users than �1 /minute.