All you need do is to conduct a Health and Safety Risk Assessment, and you are off the hook because the law requires you to do just that.

Whether or not you are fit enough to do the job, is irrelevant, if you consider the job to be unsafe, which it is.
Go to the HSE website and download a risk assessment form or knock one up. It's a piece of pi** and keep it in your taxi, and if anyone complains, hand over the form with no further comment. The council or the Cab Inspector is not allowed to second guess your assessment of the situation. You are self-employed, and it is up to you to determine what is safe or not, just make sure you follow the correct procedure by conducting your own risk assessment.
http://www.hse.gov.uk/risk/fivesteps.htmGame over...................
