Neither their special relationship nor an extradition treaty that the US and Britain signed in 2003 has made extradition easy for the US. Between 2004 and 2012, out of the 134 extradition requests the US made, Britain only allowed 75 extraditions to the states.
Take for example the case of Gary McKinnon, a British hacker diagnosed with Asperger’s Syndrome. Mr. McKinnon broke into almost 100 NASA and US military computers over the course of two years. In 2012, the US issued an extradition request for him to be tried for computer fraud, only to be denied by British Home Secretary Theresa May. Her reason: Because of his diagnosed Asperger's and other illnesses, extraditing McKinnon “would be incompatible with [his] human rights.”
Hackers aren’t the only ones who have found legal shelter in Britain however. It took eight years to extradite radical Islamist preacher Abu Hamza al-Masri to the US for conspiring to train terrorists in Oregon and for kidnapping Americans in Yemen. Once again, human rights concerns stood in the way. Mr. al-Masri made appeals to the European Court of Human Rights, which has jurisdiction over 47 countries, to deny the extradition request on the grounds that facing prison time in the US would be a violation of his human rights.