Can my friend and I spend the summer in Canada (6 weeks) at 16 years old without an adult?

Yes, this is possible. In addition to a valid passport and an Electronic Travel Authorization (eTA), or visa (if you are not UK citizens/visa-free nationals http://www.cic.gc.ca/english/visit/visas.asp), from https://www.iatatravelcentre.com all children irrespective of nationality traveling alone are strongly advised to hold a consent letter signed by a legal guardian or parent with legal custody, containing:

• authorization to travel (with another person, when applicable) and to be outside the country;

• destination in Canada, also indicating length of stay;

• the signor's address and telephone details.

The Government of Canada provides a sample.

Airlines typically allow 16 year olds and over to travel unaccompanied but to be on the safe side you should check what the airline’s policy is before booking. You should also make sure you have proof of sufficient funds for the trip to show to Immigration Officers on arrival, and adequate travel insurance.