I usually round up the fare when paying a London taxi - if the fare is £9.50 I'll make it £10.00 but if the fare is £19.50 I'll probably make it £20.00. But I don't have a hard-and-fast rule and tend to do what is easiest for both parties at the time. I have known a London taxi driver refuse a tip when it would have make the change more difficult, and some drivers seem mildly surprised when offered a tip.