Both the clubs I send my son too were recommended by work colleagues who sent their own children there over several years. other parents recommendation is probably the best place to start.
Otherwise look at your local councils website, they have childcare providers listed on there including holiday and afterschool provision.
My son goes to two different holiday clubs, one is only open in the easter and summer and the other is open every school holiday including half term and even xmas but it doesn't open on non pupil days as they vary from school to school.
I can recommend both but of course it all depends what part of the country you are in. The easter/summer one is called Barracudas,
www.barracudas.co.uk/
and they have lots of places all over the country so look on their website to see if there's one within distance to you. It more sports based and gives him a chance to try lots of new activities and he comes home pretty tired after a hard days play.
The other is seymour house which run holiday and afterschool clubs attached to their nursery and which is based in essex
www.seymourhouse.co.uk/
They offer different activities and themed weeks, bouncy castles, computer games, activities...
Holiday clubs can be quite expensive, but remember it doesn't work out that much when you average out per week over the whole year so try not to think of it as being £35 a day but work out what that will coast you a working day averaged out over the year.
Plus if you are a lone parent on WTC & CTC you will probably get up to 80% of the cost paid. So a £35 a day holiday club place (which is more than I pay) will in reality cost you only £7 if it qualifies as an approved childcare provider for CTC purposes. Make sure they are ofstead registered and you'll be fine.
Some places also offer early booking deals and 3 and 5 day deals so it's worth getting in there early if you can.
Decide what kind of childcare best suits your child and then find a holiday club that suits them.