The press release mentioned 10 aircraft being purchased. So, one could assume more than one North American destination from LCY. Perhaps Toronto, Boston, Washington and Philadelphia with a few cycles for certain destinations (NY and BOS).
Not sure LCY has the ramp space for 5-10 more aircraft the size of the C-Series (certainly bigger than the FK50). I agree a huge challenge would be overcoming BA's loyalty points programme - all business types want to get points to take their family on holiday. That's a huge market entry barrier.