An offence under Section 3 of the Theft Act 1978 ('making off without payment') is theoretically punishable by a custodial sentence but the chances of that happening for a first offence, where the sum was less than £20, are completely NIL.
Offences under Section 5 of the Public Order Act 1986 (as amended by Section 57 of the Crime and Courts Act 2013) can't result in a custodial sentence.
If you had provided the minicab driver with your name and address, and invited him to sue you, your defence of 'not acting dishonestly' might have stood up in court. Assuming that you didn't do so, I'd strongly advise you to plead guilty to the Section 3 offence (because the court will find you guilty anyway).
With regard to the Section 5 allegation, in order to obtain a conviction the CPS must prove that your words or actions were threatening, abusive or disorderly AND that they were likely to cause harassment, alarm or distress. It is not a criminal offence simply to swear at someone and (since the 1st February, when the law was changed) it's not illegal just to insult someone even if that insult is of a racial nature and is likely to cause harassment, alarm or distress.