I work in the hospitality trade here in the UK, and the tips left to my waiting staff average out at a little over 6%. (This could, of course, mean that my staff are rubbish, but there you go.)
There is just not the same tipping culture here - and you know what? I think we actually have it right.
Why on earth should I, as a customer, have to leave cash behind for the waitress after a meal because the employer pays crappy wages? It shouldn't be up to me to subsidise his business. Pay the staff properly and then tips can be more discretional, as they are over here.
The very idea of leaving a tip is surely to reward or (for those who tip before the meal [a good idea by the way] ensure better than average service.
If your service is poor, tell the manager and don't leave a tip.
If your service is OK, I would say, don't leave a tip - you have a right to OK service and just because you know that the waitress is on low pay does not make her a charity case.
If your service is very good, tell the manager and leave whatever tip you feel is right (don't let anyone tell you it should be 10% or 20%, it's up to you).
Having said that, never leave a derogatory tip of say 13p in change unless you have a death wish.