Hi Keeley,
How long is her hair?
Is it permanent or demi permanent color, that you want to remove?
If it's long the ends will be the hardest thing to strip. So start there, remove the end color first and keep the color remover off of the base.
What condition is her hair in?
Does she have any new growth and is she a natural level 6?
I like Rusk Eliminate (I don't know if it available in the UK). There are some great color removers out there...Goldwells is great. What color line do you use....they may carry one.
You will have to stand over it and watch it...and maybe even apply color remover more than once keeping an eye on the condition of the hair the whole time.