Me. I was at a barbecue in Dulwich and looked up when I heard engine noise suddenly increase. (I see and hear aircraft overhead there all day so I'm used to what is "usual" and what's not).
Both aircraft were at the end of a right turn, seemed to be at very similar altitude and extremely close laterally. I've never seen any so close. Second, faster, aircraft suddenly powered up, dropped right wing and turned very sharply.
(I know this sounds like the ramblings of a civilian, but believe me it was extremely unusual.)