Q.Weeds in boxwoods
How do I kill weeds that are taking over and growing up through my hedges! They’re fairly large and just about impossible to get under and “pull” them. Anything I can spray with!

Your Boxwoods look beautiful!
I really would suggest trying to remove the weeds by hand. Reaching the weeds from along the soil line and making sure the entire weed is removed.
Boxwoods are an investment and to risk harming them with a weed killer may be very risky. Once the weeds have been removed I would mulch around the base of the plants to inhibit regrowth of weeds.
I have listed a link that may help. Perhaps using the cornmeal would inhibit the weeds from seeding again.
It may be worth a try.
https://www.gardeningknowhow.com/special/organic/cornmeal-in-the-garden.htm