I've been working freelance for about a year now, I done three books for personal clients alongside other commissions. I've taken a low pay for quite some time. I'm now considering getting an agent to help me reach my goal of working on bigger published books.
Your work looks pretty good, but you have to do a lot more research. Start by spending some time on Google searching terms like "children's illustration agents". I can't offer you any personal recommendations because I don't have an agent and I'm based in the US. If you were here, I'd recommend you join SCBWI. Perhaps there is an equivalent organization in the UK. Try to go to some industry trade shows (whatever is the UK equivalent of Book Expo America). Try to get appointments to show your portfolio to publishers, and while you're there ask if they can suggest any agents. Directly contact a few of your favorite artists and very politely ask them for advice. If you went to school, ask your teachers. Learn everything you possibly can about the business of children's illustration and begin to make as many connections within that world as you can. All this will help you find the right agent, or who knows, you might find you don't need one.
You should also be aware that most publishers prefer to match artists and writers, so any spec work you do with a writer to illustrate their book in hopes it will be published is actually counterproductive (except to whatever extent it may be helpful for your portfolio).