You asked

How many people were admitted to an NHS hospital with the main reason being a UTI during 2015?

We said

Thank you for your request. The Office for National Statistics does not hold information regarding hospital admissions. We suggest that you contact the NHS Health and Social Care Information Centre (HSCIC) to see what information they are able to provide: http://www.hscic.gov.uk/contact-us

The Diagnosis spreadsheet within this link may be of interest: http://www.hscic.gov.uk/searchcatalogue?productid=19420&q=title%3a%22Hospital+Episode+Statistics%2c+Admitted+patient+care+-+England%22&sort=Relevance&size=10&page=1#top