The 10% surcharge is sufficiently rude that I won't tip. No credit card company charges anything like that as a fee so it is purely to fund Dialacab (and partly offset the driver having to pay tax on that bit of income too ).

Not sure I'd consider 10-15% a normal black cab tip. I normally round up to the nearest pound (or the nearest plus one over £15) and am normally thanked genuinely for it.