Whilst some routes might migrate from LGW to LHR, I am sure IAG would want to retain a presence at LGW for future expansion given the current stalling or R3 at LHR. It's the same reason BA maintain a presence in T3 at LHR- it makes flexing and growing capacity easier because you have an established infrastructure. With added slots at LHR, BA may be able to achieve medium-0term growth, but long-term LHR will always be constrained.