I traveled out of the country several times with my niece prior to her turning 18, including to the UK. You should have notarized consent from both parents for the travel and for you to make health care decisions on her behalf. I never had to show the letter but on one occasion at the border I was asked if I had a letter of permission from the parents. The one I used was adapted from several ones freely available on the Internet.
Info from CBP:
https://help.cbp.gov/app/answers/det...ot-a-parent-or
https://help.cbp.gov/app/answers/det...RkVHdTbA%3D%3D
Alaska Airlines recommended consent letter:
https://www.alaskaair.com/~/media/Fi...f-Consent.ashx
Canadian recommended consent letter:
http://travel.gc.ca/travelling/children/consent-letter