I swear by Autoglym Bodywork Shampoo Conditioner. Available from Halfords in
this link.
You can also get it in smaller bottles which are cheaper. However, I buy it in 2.5l bottles, one bottle lasts me over a year and I probably go through 4 washes a month between the 3 cars we have at home. Just fill the cap 3 times and tip it in your bucket, that's all you need because the Autoglym is highly concentrated.
The difference between this and cheap shampoos is that it is PH neutral, making it friendlier to your bodywork. In addition, it won't strip any wax you've applied (as long as it's a detergent proof wax... but then again it should be!!!). Other quality shampoos are also PH neutral, however a lot of supermarket shampoos aren't.
In addition, I've found the Autoglym to give a better shine than the cheap shampoos you get from Asda and the like. It's well worth the money. Highly recommended by me.
Oh... and you best be using the two bucket method followed by drying with a proper drying towel.