I don't know the exact spec on the 6400, but if you can, I'd try to stretch a little more and get the A7III for £949 at cameraworld. It's just all-round better than the earlier A7 series (better battery life, ergonomics, eye tracking, colours etc).
But otherwise...
Do you like shots with shallow depth of field, low light photography at high ISO, like some 3D pop in your images? A7
Do you need your kit to be small and light, prefer a bit more depth of field in photos, take pictures in brighter conditions? 6400