Yes indeed good luck for new route, I'd pose some challenges to comments both negative and positive ;
Yes LCY has been tried before and Gazmo , you say it has never been popular ex NI. I'd say it's never really been a credible option to make it popular. It was operated by small Dornier the same season as FR and EI launched Belfast , the seats were crazy prices about £300 at times and didn't have particularly a good timetable. BE did it before via IOM and wasn't particularly a convenient option to get to London .
So we cannot really judge it until a few seasons of a solid timetable to judge. If GLA, EDI and DUB can support a London City route no reason why BHD can't .
On the flip side I say that LCY is absolutely not more convenient than LHR or LGW unless you need to be in the area of Canary Warf/ nearby areas. Travel time to Central London ie Victoria, Piccadilly more easily and quickly reached via LHR or LGW for that matter. Has anyone ever had to wait on a DLR and then stop everywhere to get to Central London ... ?
I've done LCY many times ex dub and to a few destinations further a field , I like it on balance ie time and price etc - I don't see it as preference no 1.
The prices by BE are lead in fares, as bookings are only starting , let's see the pricing on the flights when they get going. They will hit BA for there business pax, I'd suggest EI is not as dependent on business traveller as BA is...
Interesting times ahead