With the new deliveries of A320/A321NEO aircraft to LHR in recent months BA has enough A320 aircraft to cover all flights this summer using its own metal. I believe they have taken delivery of at least 12 new A320/A321 aircraft and only retired 2 A319.
The 777 fleet at Gatwick will also be increased to 14/15 aircraft. However like the A320s the G-YMM series seem to interchange regularly these days. G-RAES has also become a Gatwick regular recently.