If it was proven it wasn't London at all, basically she could have been anywhere, not even UK as there were several places with clouds where rainbow could be seen (like Kika in north of France)
(Also, cloudy/rainy weather doesn't mean there would be a rainbow as well)
Maybe she doesn't post something specific as Lando has basically disappeared and if him or noone else posts specifically where he was, she cannot have it linked to him, it's all about rumors or speculations only. If she posted something specific and later it would be proven he wasn't there, she'd look like a fool (again).
(But I love how she denies anything Lando but implies she has something to do with Lando which essentially makes her a liar and then she wants people to believe she wasn't cheating. How many brain cells does she have really?)