Address in the UK before applying for a visa?

I read on a UK-canada immigration info site taht you need to have an address in the UK prior to applying for a visa? How would you go about that if you dont know anyone you can stay with for a while?

I just applied for my Ancestry Visa (going for biometrics next week and should know soon on the outcome) and I chose hotel/b&b under the options for the "where will you stay when you arrive?" question. I was not asked to provide details of the address of the hotel and I am going to put something (i.e. a print out of the hostel I am staying at) in my back-up documentation.