This yellow comes from tansy (Tanacetum vulgare). Tansy is common British specie and for ages was used for different purposes – not only for dyeing! Thanks to it containg of aromatic oils it is very good bug repellent and was used to scare away mosquitos and flies, as well as moths. Bunches of tansy flowers were used in the Medieval Ages in a similar way to way to which lavender is used nowadays – they were put in chest between clothes to prevent them from being attacked by moths. Maybe this is how the plant become dyestuff – as a result of somebody's attempt of making wool moth-safe?