Wycombe - not saying that HAH would want Flybe to fail. More that in much of Surrey and north Hampshire there are a lot of wealthy people. If SOU becomes too well connected to other European hubs, the possibility of connecting in (say) Madrid reduces the number of people who might be interested in a LHR-LIM nonstop route.
I'm wondering if it's perhaps in HAH's interest to put some sort of limiter on SOU's growth on routes to major hubs, encourage just routes to UK/Ireland/beach/small French airports instead, and retain all the passengers potentially travelling long haul to go to Heathrow instead. In effect, HAH keep all the cream at LHR but dump the less desirable offcuts at an outstation.
Of course whether HAH actually act in this way is a separate question entirely.