Riley,
The P8 is rather faster than the Sea King therefore there is no need for dispersed flights. As already opined, Waddington is central and sensible as the P8 is yet another ISTR asset.
While the Nimrod was originally based at St Mawgan and Kinloss I suggest that was both for historic reasons (give CinChan his own air assets) and force numbers. Kinloss could not have handled 4 sqns plus the OCU.
Your north-south suggestion even in the 70s was invalid as we often flew SWApps Ops from Kinloss and loaned assets to SouMar to prosecute as far as Lajes.
Also, by the time the P8 could be introduced to Waddo it is likely that the Sentinel will be on its way out.