Can't really see why this is any form of slight towards SEN. I don't know the full details but these must be routes that BA wish to operate and Stobart have secured a (I assume) valuable contract to provide a full ACMI service. Had Stobart not done so someone else would have.
So BA have plans to expand at LCY. Well, nothing SEN can do could prevent that. And it isn't as if Stobart have pulled aircraft out of Essex to work for BA. Clearly, they have sourced the aircraft specifically for this work alone.
And you never know, all this may enhance the prospects of a couple of BA aircraft being kept busy at the weekend at SEN.