Appropriate tip for a taxi is nothing. The fare has been agreed, the driver does his job and takes me from A to B, perhaps a few wise cracks along the way. Inevitably I retrieve my luggage from the boot. The driver has done nothing more than a bus driver, tube or train driver or even the cabin crew on my flight to my destination who arguably have done much more in terms of service than the taxi driver but no-one ever tips them.
Many a sparky black cabbie has quickly changed his tune when he's realised he's not receiving a tip which just goes to show that it's all a charade and he couldn't give a toss about you, your life or your plans.
I never received a tip as a software engineer, a techicnical support techicnician, a police officer or a market trader. The only tips I were offered was when I volunteered for my local Age UK group where I would serve lunches to the lonely elderly and provide them with basic computing skills, of course I always declined such tips.
Tipping deserves no place in the UK otherwise we'll eventually see it turn into the mess that is US tipping culture.