You're talking about a winter timetable - so I presume you're looking to book flights after the end of October?
Do bear in mind that winter timetables haven't yet been finalised - they will be finalised in around August. What you're seeing in the system at the moment are (automated) extensions of the summer timings, except in those cases where a decision has already been made (as will almost definitely be the case with the fourth NWI flight which has been a summer-only service ever since it was introduced four or five years ago). Two a day to SOU is the norm all year round, and KLM flights to/from LCY have always been thin on the ground outside the morning and evening peak.
Have you tried booking your flight as an open jaw? I've booked open jaw where one leg terminated at AMS, but it's been the outbound (say - LCY-AMS, HAM-LCY), I don't know if you can do the same with the return. If not then then the only options I can think of are separate tickets (though I wouldn't recommend that), or travelling with hand baggage only and skipping the last leg on the return.
All very complicated - if that was me I'd just go for the five hours, it's not a big deal.