I have written 10 books. Yes, finished 10 books of 16 in a series for kids aged 5-8 roughly. Been compared to Spongebob (works on 2 levels). Cartoon illustrated books. Merchandising potential is for 30 characters in the books. I am already published. Not self-published. I would like a UK based Agent as I also have a crime novel (finished) and a sci-fi novel half way written. I would prefer a UK publisher. Advice? Should I get ISBN as first book will have covers on back page for "Also available in this series..."
I wouldn't buy ISBNs unless you are thinking of self-publishing. The fact you are already published means you have relevant experience to show and have an excellent chance of landing an agent, especially with the works you seem to have ready to go. I would suggest looking for agents who specialise in children's illustrated fiction but whose agency also represents authors of other genres. There are a few in the WAYB.
The difficulty maybe finding one whose prepared to look at the sci-fi as well as the children's books. Look carefully at each agency and only submit what they are asking for. For instance, Celia Catchpole only deals in children's writing and illustrations so might not take on an author diversifying into other areas. The Caroline Sheldon Agency does take all forms of fiction as well as specialising in children's illustratated books so on the surface they might seem like a good idea (especially as their website is so helpful) but more often than not they take on women's fiction and aren't often drawn to crime and sci-fi. It's going to take quite a bit of research to find the agent prepared to handle all of your work and your pitch will definitely need to be individually tailored to each agent you try based on their preferences. Fortunately, once you find their names in the Writers' & Artists' Yearbook, it's not too difficult to google each one and find all that out.
Good luck.