People sometimes think it is civil as they view it as a contract, between the driver and the fare, and i can kinda understand that. However, there are actually two criminal offences to be considered here, or at least there is in NI, as I'm not sure the theft act is entirely the same on the mainland? Firstly, as mentioned above, you have the 'obtaining services by deception' type offence - if the fare has no money, then they can not pay, which is what is expected of them. You also have the option of the fare 'doing a runner' whether they have the fare or not, which is slightly different. It all depends on the attitude of the local fuzz. I guess it could be pretty common, a bit like drivers not paying for petrol, so their experience will dictate how they proceed. The easiest and best solution is always to get the money, but this isn't always the case. The bottom line is that an offence has been committed under law, and police can deal with it (whether they like it or not!!). I don't envy taxi drivers, and take my hat off to you guys! Sometimes though, taxi firms have their own manner of dealing with these things, as mentioned, and I suppose you can get away with anything once!?
Just my tuppence worth.