The UK's laws regarding citizens arrest are pretty much the same with a few exceptions:

  • The key act in the area of citizens arrests is the Police And Criminal Evidence act 1984 (PACE)
  • It doesn't matter whether the crime was committed during the day or the night, you can do it any time you want.
  • There is no limit on how petty the crime is - if it's a crime then you can perform a citizens arrest for it (whether you will get prosecuted yourself for false imprisonment or assault/battery is another story...)
  • As in America, however, you will be arrested/sued if a crime has not been committed. It's only the police that can arrest on suspicion that a crime is about to take place. A good example of this is the 1914 case of Walters v. W. H. Smith (yes, he of stationery chain-store fame) - he was working at his stall, selling books as usual when he noticed a customer slipping a book into his trousers. W. H. Smith then proceeded to conduct a citizens arrest, but the jury believed the defendant's plea that he was simply holding it there until he was going to pay for it and subsequently acquitted him - W. H. Smith was liable for damages. For a similar reason, store-detectives cannot arrest you until you have actually left the store. But I digress.
  • You do not have to have witnessed the crime. You must only have reasonable suspicion that the person is guilty. However, more fool you if you put your clean criminal record on the line to arrest someone whom your friend claimed stole something.
Do not perform a citizens arrest. It is really not worth the risk.

This info was all taken from what I have learned so far studying English Law at college.