Your employment contract stipulates your weekly hours. 37.5 or 40 is “full-time”.
The maximum contracted hours per week possible is 48. You may work more, but you have to sign a legal document declaring your desire to. It is illegal for your employer to treat you in any way unfairly for refusing to work more than 48h/w. You can cancel this agreement with 7 days notice. Again, it is illegal for your employer to do anything about this.
anything over the contracted hours is “overtime”, which is paid at a rate negotiated with your employer.
Some places have very generous overtime rates, others pay nothing above normal wages.
There is no regulation of overtime wages, beyond the set minimum of £6.19 (as of october 2012) per hour, which is about $10. Tips do not “count” towards your wages. Every adult is legally guaranteed at least £6.19 for every single hour worked.
Young working-class people will start out at or near the NMW.
More experienced – but unskilled – factory-workers generally earn £8 or more (~$13), with higher pay for people with more skills i.e. setters might get more like £12–15 ($19~24)
Wages are higher in London and The South, and lower in The North.
Prices and the cost of living, however, exhibit this trend to a greater degree.
i.e. a pint of beer is costs fewer local-man-hours in Sheffield, than it does in Reading.
The situation in offices is similar (though overtime is much less common). Wages are about the same as in the factories for most people, though obviously there is much more room in the office for progression, for those with the ability and inclination.
National Minimum Wage applies neither to apprentices (who are usually young people, living with their parents whilst learning a trade), nor to self-employed people. So for a self-employed tradesman (many of them are) you can earn £100,000 in one year, and literally nothing in the next, as happened a few years ago to my father (a sparky).
The only exceptions I know for the working-week limit are emergency services (plus armed forces)... and fishermen… who can’t exactly be said to work 9–5.