I like to leave a little tip an extra £10 or £20 if its someone i want to return to in the future (and they were darn good too)
Think of it this way. A girl is on £100 plus per hour. Do you think she will think you are special for giving her £10 ontop?
What do you think is going through her mind when you say, "you know that was great, you know what here is an extra tenner ontop".
Do you really think it would be along the lines of "wow what a lovely generous guy". Or do you think its along the lines of "Christ a tenner, is that all he thinks I am worth, a bloody tenner. Fine at least I have something to wipe my arse with tonight".
Buy her a bottle of vodka or some chocolates for that tenner is my advice. It will come off better.