Goldfinch sounds likely, they are certainly numerous in London. Can you find a link with their song, I think YouTube has some.
Butterflies not bad right now, presumably the weather is helping. Have seen painted lady, comma (looking very orange), gatekeeper, ringlet, skipper, meadow brown, holly blue, forget others.
Also saw today a nice fat hornet, who was enjoying the cat poo. Haven't seen one for ages, reminded me that when we were kids, we were terrified of them, I suppose their sting packs a punch. Apparently, they use attack pheromones to recruit defenders of the nest.