And the Go 737's were not mainline aircraft?
No, they were new orders for a separate low-cost carrier. BA European is not intended to be a "low-cost" airline. It is planned to offer the full BA mainline product.
CityFlyer operates an aircraft that has been operated by mainline at Gatwick.
Correct. The RJ/146s were inherited from the original Cityflyer and absorbed into EOG. They were subsequently transferred up to the regions
with mainline flight crew as the Airbus and 737 fleets were withdrawn. They were then transferred to BACon, with some secondments and were retained by BA when BACon was sold to BEA. These aircraft are operated on an alleviation to the sub-100 seat Scope agreement and will be disposed of in favour of smaller aircraft when the leases expire. Some seconded Captains remain.
Like I said, mainline product, mainline aircraft so mainline pilots.
FWIW, I don't think there should be any requirement for Scope. All BA flying (the sadly demised BA Connect, BA Cityflyer, BA World Cargo and BA European) should be flown by pilots on the mainline seniority list.
