This weather is bl**dy annoying isn't it? One minute I'm freezing, the next I'm sweating like a pig. It's borderline impossible to dress, but the one thing I've found works is sleeveless, long length dresses. You can pop a shirt underneath it (this one from ASOS is a favourite at the moment), throw a jumper over it (ideally, this one) and then when the sun comes out, chuck them off and enjoy the 15 minute heat wave...

checked dress, Whistles / white trainers, Adidas Originals at Net A Porter 

The ones I really love are checked, like above. You can pair them with heels for evening, or trainers for daytime (this lady has got the red Stan Smiths I'm after, find them here). I spied one at Whistles, here and have had the need to buy one instilled in my head ever since. Having done some research, I've found a much cheaper version at Dorothy Perkins here so you don't need to break the bank...