As a retired Broadcast Television Engineer and VT Editor with extensive experience on sport outside broadcasts as an EVS operator, I am puzzled as to why you have to pay extra for HD. All OBs are in HD these days and if there is to be an extra cost it is for the extra signal processing involved in downgrading the signal to Standard Definition or SD. There is no way to meaningfully upgrade SD to HD. Another rip off?
The HD channels use more data when transmitted over the broadband based multicast system (TV Connect) set up by Openreach with space on it rented out to providers by BT Wholesale. I believe providers are charged per channel based on data usage? So I would expect that an HD channel will cost BT Consumer more to deliver than an SD channel. Also the HD versions/costs are in addition to and not instead of SD versions/costs until we all go fully HD only.
Does it all add up to £4 extra per HD customer? Who knows?
BT could just spread the extra cost across all customers by adding a quid or two to every TV package, but some would say that is not fair on those who currently don't want HD.
The charging model of charging extra for HD channels has been estalished by other providers and indeed by suppliers prior to BT's entry into TV broadcasting.
As umpire says the extra data used in distributing HD programs is probably a contributing factor - plus the cost of deverloping , testing and implementing an end to end process capable of maintaining the quality of a HD IPTV would need to be recouped.