Can't see BA metal on the LGW route, didn't they pull MAN not so long ago. I would imagine that BA will replicate their codeshare as per LHR from Belfast, allowing for connections. I could also see VS codeshare also.
Longterm, I think rather than EZY vs BE, it'll be EZY vs EI, albeit with a bit of capacity shuffling. EZY may up frequency initially to steal market share, EI might add a fourth rotation, but ultimately it'll reach equilibrium just as it was with EZY and BE competing.
BE were carrying about 22k pax per month at most on the route BHD LGW. EI will be keen to capture all of that level of business for themselves, and I would expect they will hope for x3 daily well subscribed 319s and get some yield on the market. However, they would need to improve the timing of the last back flgiht to ensure good for business as well as current leisure passenger.
I would suggest that EZYs gains have been many ex BFS LHR passengers who want to use BFS as their Belfast airport.
BA would be better to focus on improving their performance on BHD LHR as with the high frequency they and EI combined are only at loads of 50 something % per month, and BA have had good value too so it would suggest yield has not been spectacular.
EI-BUD