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.