The black cabs that do take credit cards generally have Mastercharge/Visa/Amex decals down the side, so you can hail one on the street, or be certain at the airport, if you see this. It's not all but it's quite a proportion.

It's a nuisance way to pay this way though because the technology depends on a mobile phone auto-connection from the credit card machine in the cab to the verification centre and it always seems to take ages for this all to happen. The surcharge sort of compensates for this.

BTW, London cabs have all sorts of ownership. Traditionally they were owned by their drivers who worked independently and drove home at the end of the day. Some are shared between two drivers. Some don't own them but hire them either by the day or on longer term arrangements. Some have radio dispatch or belong to the various booking companies like Computer Cab, which is like an association they belong to rather than ownership in many cases. It tends to be the independents who have no radio and no CC facility.