It has been said elsewhere that the 10 A320 with sharklets will eventually move to Gatwick along with extra A319. I expect by late September BA will be operating nearly all services on its own aircraft. By that time a few NEO Airbus will have been delivered to LHR.
Nothing unusual about airlines leasing in other aircraft during the busy summer months in Europe. Remember BA only purchased the slots in November 2017 so it will take time to service them all with it’s own fleet.