h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

FX4 is not a fairway
The FX4 was originaly manufactured to carry four passengers, it was fitted with the 2.5L Austin engine. around 1984/5 carbodies fitted the 2.0L Rover engine, it would'nt pull the skin off a rice pudding!!!
So around '85/6 the engine waas upgraded to the 2.5L rover engine and caled it the FX4S. This production run of vehicles benefited from an early form of rust proofing, the engine leaked oil so badly the whole chassis was coated wit oil.
In 1987 Metro-Camel produced a fiberglas bodied taxi with a Ford engine to comply with the London Conditions of Fitness, This was a five seater cab and was also wheelchair accessible (in order to comply with coming legislation).
In response to the Metrocab, Carbodies made substantial modifications to the bodyshell of the FX4s and called it the FX4Plus. This made the four seat FX4s a five seat cab, but this was only a stop-gap! The enthusiastic and revolutionary designers at Carbodies were planning a brand new vehicle, the 'Fairway' outwardly it looked identical, however few of the body panels were compatable without modification. The rear doors were completly different and opend much further, this allowed for the carriage of wheelchair bound passengers, the most noticable difference was the internal partition, the original cabs had a straight partition between the two door pillars, the Fairway had a cranked partition to make a recess into which the wheelchair was secured. The cab also benefited from a Nissan 2.7L engine, which didn't leak!
So the FX4 isn't a Fairway
In 1992 The Public Carriage Office changed the conditions of fitness and approved the fitting of ball joints instead of King pins on the front suspension and also Disk Brakes. The official line for not alowing disk brakes was "It is a taxi, not a racing car"
The fare structure in London Taxis is now set by the G.L.A. and not parliment. this was changed when the Greater London Authority was formed and Transport For London was incorporated as a body within the GLA.
The anual plating of a london taxi is stringent, however it is not the conditions of fitness.
The conditions of fitness are a set of rules defining the structure of the vehicle (monocoque construction is not allowed) the size of door openings, height etc. It runs to severeral hundred pages in two volumes and is concerned with the fitness of the type of vehicle that can be licenced to ply for hire in London, part of the reason that a new cab costs over £30K. For example A Fairway that is licenced to ply for hire in Glasgow would not meet the conditions of fitness for London.
The anual test, or 'Overhaul' as it is known in london is similar to the MOT test on a car or comercial vehicle but much more stringent, the official line from the PCO is that they cannot fail a cab on anything that would pass an MOT test, including the tyres, however they will fail a cab for a missing spare tyre, faulty or missing cigarette lighters, interior lamps and unaproved radios!
and in response to someone else's comments about miserable cab drivers at Heathrow, believe me they are pretty well all miserable, re. above reasons.
One more thing concerning the 'Knowledge'. When the knowledge goes in common sense comes out.

Thanks for all this additional information.

My dad is in the Guild of Hackney Carriage Drivers in London, and I got a lot of information from him.