Marrying on a Tourist Visa would be viewed as an abuse and create problems later. It a is also doubtful a Registrar will allow a Ceremony. Others may have a different opinion but probably your best solution would be leave UK apply for a Marriage Visa from the US, this allows you to come back get married and then you must leave immediately. If you intend to return to Uni (in US?) this is probably the best answer. Others may be able to add or suggest otherwise.