We had our child's party at the South London Scout Centre, SE21, email: firstname.lastname@example.org
We booked the Sunley building, which is a massive space with a well-equipped, quite large kitchen and toilets. There are also fold out tables and chairs available at no extra charge, lots of outside space and free on-site parking.
We thought the cost was really reasonable (£50 for 4 hours, though this was the cost last year; it may have increased), and everything was much more impressive than we were expecting, given the few photos on their website.
Something to keep in mind is that you will need to decorate and set things up yourself.