SEN is well used to handling LCY diverts over the years and its a fairly regular feature in bad winters, even before the new terminal etc, so in general its not a prob.
Although a lot of regular LCY pax are used to being off the plane and through the terminal in a matter of a few minutes so any delay and train journey more than 10mins will seem 'hours' to those people.
SEN is probably the best option for LCY diverts, but agree its not something to base a business case on. But I've often wondered if BA could schedule some of its Saturday LCY arrivals into SEN (1 or 2 end up there most summer weekends) which would then free up those a/c for a further 1-2 saturday afernoon / Sunday morning rotations, maybe for a tour operator?