Hello Ruth
We live in Woodside Park, so know your area well.
The person who responded last is quite right, it depends upon whether you are looking for day or residential camps.
One of mine goes to a residential camp run by two local brothers. It is relatively small, fills up very early each year. It is marked 'Outstanding' by Ofsted, which is relatively rare and we find it unique in what it gives for our children. It is called XUK (short for Experience UK) and is really aimed at teenagers, although some send their children from as young as 6.
They also run day camps in your area - one in East Finchley and one in Hampstead Garden Suburb. Both are fairly well known and have an outstanding reputation in the local area (Mini Minors and Cross Keys).
Their website is www.xkeys.co.uk and I hope you find them as good as I do - good luck.