How do you pay the fare and include a reasonable tip without making it a "OH BY THE WAY HERE IS YOUR TIP NOW KTHXBYE" moment ?
Edit: I meant to include situtations other than taxis - e.g. in a restaurant, paying by card with one of those nifty portable card readers, except it's one of the models which doesn't allow you to enter a tip - but you've already gone past the point where you can say "just make it £x" to the server. Do you just leave coins on the table afterwards ? What if they reason you're paying by card is that you have no cash handy ? Slink away, figuring that if they want your tip, they can darn well make it easy for you ?