I've done South-East England to Cannes Mandelieu many times in various SEPs. Average flight times are around 4:30 according to my logbooks.
Bladder management consists of the usual precautions about not drinking 2 litres of sparkling water ahead of time, and having some good alternates available en route.
For me, the enroute stop-offs to be used in the event of bladder pain were Le Touquet, Reims Prunay, Dijon Longvic, and Lyon Bron. The two middle airports require a nominal PPR for customs. In the event, this can be flexed slightly if you "need" to land. Much better not to put that to the test though.