Looks like one of the Hawksbeads possibly the common one [Crepis biennis.] How tall is it? this one grows 60-120cm 2-4ft.

Does it have a milky sap?

Similar tro our Sow Thistle (Sewan). A weed in gardens.

yes it has milky sap. It's about 18 inches high. It's in my butterfly garden. Should I leave it for them or pull it out? Thanks

yep Bulba, Sow thistle is a better match going by the buds .
Do the leaves clasp the stem? is the bud hairless? if yes then it is sow thistle Sonchus oleraceus. 30 -100 cm tall 1-3ft.

It self seeds freely and was eaten as a veg in medieval times. I must admit I pull it out as if I don't it soon goes on the rampage. the sap can be an irritant on sensitive skin if exposed to sunlight.

its a good insect plant for a wildlife area.

Thanks, so I'll just leave it alone. It's near my tomatoes so maybe it will draw pollinators.

yes it will the hoverflies love it.

If you leave the seed heads on you will have rather a lot next year though

Thanks Ster this weed is destined for deadheading. 1 is plenty.