Tattooing a person under the age of eighteen shall be an offence except if it is performed by a duly qualified medical practitioner or by a person who works under the direction of the duly qualified medical practitioner for medical reasons and is done for medical reasons. A person charged may also be able to show that when the tattoo was performed, he had a reasonable suspicion that the person tattooed was at least eighteen years of age and did believe that at the time the tattoo was done. So to answer the question, you cannot get a tattoo at 17 in the UK.