Operations on parallel runways are dependent or independent in either mixed or segregated mode. This is covered by the SOIR rules ICAO Doc 9643.
Heathrow was built before these rules were developed and the two runways are so far apart that under SOIR they can be counted as independent in all modes of operation.
Therefore, leaving the Cranford Agreement to one side, ATC could treat each runway as if it was a separate airport. Clearly issues about landing runway and final terminal destination revolve around which hold arrivals are picked from and hence how to control traffic on the northern runway heading for T4 and vice versa.
Nevertheless. If Gatwick single runway ops can deliver over 50 per hour on one runway that kinda points at 100/hr at LHR. Or am I missing something?
SGC