The basic rule in the UK is that no worker can work more than 48 hours a week on average. The average is usually calculated over a 17 week period, meaning that if you go over the 48 hours on some weeks but are under on others then you may be ok.
However nothing is ever simple or straightforward. As an employee you can choose to “Opt out” of the maximum 48 hour rule, but you will need to sign a declaration before you do so.
If you have more than one job, you should inform each employer of the fact that you are working multiple jobs so that they can each monitor the total number of hours that you are working.
There are also a number of exceptions to which the basic rule does not apply, such as if you are working in a situation that is covering 24-hours a day. This exception may therefore not only apply to live-in carers but also where home carers are working on a shift pattern to look after someone 24 hours a day.
The 48 hour a week rule has come from an EU directive which was intended to protect workers health and safety. The thought being that working long hours each week can put the worker at risk.
This law also stipulates that people should have a minimum of one day off per week as well as 11 hours rest in between shifts. This is highly relevant for care workers who do late shifts followed by early shifts.
There is much more information available directly from the government website https://www.gov.uk/maximum-weekly-working-hours and if this is something you are concerned about in your employment you can also contact ACAS who will be happy to give you some guidance. http://www.acas.org.uk