Walker Books, an established and respected UK based publisher of picture books, is open to direct submissions, but only of work that is already illustrated. So if you have an un-illustrated picture book, this is not the right home for it.
Walker Books is part of The Walker Books Group is one of the world’s leading independent publishers of children’s books. Their international imprints include the vibrant international Candlewick Press in the United States, and Walker Books Australia, which has locations in Australia and New Zealand. They’ve published many of the most established writers and illustrators, and my bookshelf contains a lot of the books they’ve published. You can get a good feel for their catalog here.
As part of their submission guidelines, they link to two resources. One is Writers Advice for Children’s Book Authors. I’ve encountered them before because they publish children’s books through their imprint, Wacky Bee, but I have not reviewed them, because they heavily encourage writers to use their paid feedback services, before submitting to their publisher. I’m not saying that their feedback isn’t good, but this sets off all sorts of alarms for me, especially because they charge a fair amount for all their resources, which do seem to be personalized.
The other company Walker links to, is Writers & Artists, which also offers paid services, but gives a lot of what appears to be quality information away for free.
They accept submissions by email and post, and are open to submissions just by illustrators as well. They take over three months to respond to submissions via post (and only if you include a SASE), and they only respond to email submissions if they are interested.
Please follow their submission guidelines carefully and only submit if you feel like your work is a good potential fit.