There is the Harringay Club next to Hornsey station. I know they have week long summer activities, though I can't see them advertised on their website here.
The playschemes run by Haringey Council are excellent (no, really - they are!) You can book by the day or by the week, it costs £18 per day. Should still be some places available, there are still vacancies at the play scheme in Alexandra after-school club, and I can vouch for that playscheme personally.