Tipping the hair dresser

I've got my first hair appointment here in the UK on Friday (and boy do I need it!) I'll be getting a cut and color done. What is the standard tip rate here? I used to do about $10 each for my stylist and color people (approx 15% or so). I don't want to under tip, but at the same time it's already *really* expensive to get my hair done here!

Thanks in advance!

Someone started a thread about this a week or so ago. And there are dozens more around here too! 

Thanks, chary!