TOP ACTING AGENCIES CHILD/TEEN IN LONDON?

In the UK the only way for a child/teen to get into an agency is by getting spotted performing at their stage school's showcase. Agents get invited to attend, and if they're particularly impressed by any of the students, they might consider offering to represent them. You'd need to have been attending a top stage school or really good acting school for at least a couple of years, and have taken leading roles in a wide variety of stage plays. Your acting class or stage school should explain all this to you when you're ready for an agent. They'd also be able to recommend which agents are genuine and which are scams.

As Cogito points out, you can't get an agent until you have several years of training and experience. If you want to become an actress, you first need to start taking acting classes and performing in school drama, am dram, youth theatre groups, etc before any agent will look at you