The good news is that the market for Middle Grade and Young Adult fantasy is pretty strong at the moment. Certainly look at the Writers and Artists Yearbook, consider joining the SCBWI (UK section here http://www.britishscbwi.org/
) and think also about whether your books would work in the US market, which is very much larger.
I thoroughly endorse the advice given to get the books read by people who are able to offer critique. That many kids have loved them is very encouraging and suggests that you have good stories, but you'd be surprised by how much better they could become.