I know enough about goldfinch to be aware that they eat thistle seeds. According to a specialist birder site, the also rely on dandelions, sunflower, alder, birch, goldenrod and ragweed. That last one is a surprise - it's a North American plant that has become invasive in Europe, apparently notorious for provoking allergies. Not a olant I'm aware of ever seeing 

Knapweed flowers look a bit like thistles but are not close relatives. Most descriptions of the plant focus on the flowers and their value for insects, particularly moths. It took me a while to find someone who mentioned the seeds, confirming they are often food for finches

Amidst all the environmental doom and gloom, goldfinch are a success story - their UK population has doubled since 40 years ago. Usually shy and wary, this one was prepared to risk tolerating us quite close in order to finish his meal - subject to keeping us in sight of one beady eye