Hmm, according to VisaHQ it’s 180 days (for both Brazilians and Americans, whether tourists or students).
Also says 6 mo. here: https://www.gov.uk/general-visit-visa/overview
We’ll be staying in an AirBNB for the first week until we find a place… hopefully they’ll accept that. And we’ll have an outbound ticket - though it will be onward to another country, not back to our home country.