Looking at the timetable, RNS will be thrice daily and CFR just 4/7.
Re BE at LCY, where are the slots coming from? Putting 2+2 together, do they know something we don't about Cityjet and their future intentions -
Cityjet has axed a ton of routes recently (EDI, DND, LUX, PAD, FMO etc.), so there should be sufficient slots available.
Rennes currently has no flights to London and given the below could be why it will be double daily. In 2009,the inner population of the city was of 206,604,and its urban area counted 663,214 inhabitants. The inhabitants of Rennes are called Rennais in French. Rennes has the second-fastest growing metropolitan area in France.
IIRC, RNS has had LON flights a couple of times in the past. Brit Air to LGW and Brymon to LCY come to mind.