Marne- welcome to the thread!
This site has a list of agencies with ratings etc on them www.childmodelling.net not all agencys charge to be with them. Most of th best ones charge a minimal fee. BB is www.bruceandbrown.com or you could try www.urbanangelsagency.com www.kidslondonltd.com www.motmodels.co.uk thats just a few there are loads out there
I brought an acting and modelling guide book before my ds started modelling. I wanted an insite into it. We commute to London mainly for jobs etc and we are cambridgeshire. You can do it from all over yet have to really be commited. I am lucky we haven't ever had to turn down a job (we didn't have much work last year) Most shoots are london based. We go on teh direct train so are lucky to commute in although we can drive (providing traffic and kids are good!)
I would email a few agencies to see how you get on. Some of thems books are closed at present starting again Sep-Nov time so you may want to wait a bit