Looking at that mission London to Topsham, I'd be landing in Exeter every time. It's closer than the nearest ramp (slipway) I can see on Google Earth. Having a proper ramp is key to that job, and the attached maintenance load with saltwater operations has to be factored in. Very few ramps are wide enough, secure enough and have the space to park an aeroplane. 99/100 owners won't leaving a flying boat on the water overnight. It's not the norm. Vast vast differences between seaplane built and boating built infrastructure that only become apparent when you own a seaplane and make a few mistakes. Look up N84219 on AAIB site, just a handy example of the traps that await.