I haven't noticed that taxis (black cabs) are prevalent offenders, but private hire, now that's a different ball game. Following some of them I wonder if most didn't get their licences out of Cornflakes packets.
I had an incident a few weeks back where someone had parked half on the pavement, and on double yellow lines leaving no room for wheelchair users / mothers with pushchairs to get past. I 'accidentally' brushed his wing mirror, and was regaled by a tirade of abuse from the driver, so I got out my phone and asked him if he thought perhaps we should get another opinion from the Police. He didn't wait around to see if they turned up or not.
Watching a Police Motorway program last night highlighted the issue, immigrants and visitors to the UK can go and hire / buy a car and drive it without even glancing at the Highway Code, the Eastern European driver they stopped had absolutely no idea of the national speed limit in the UK.