What to show as proof of accommodation when applying for a UK visitor visa?

You should not submit a hotel booking with your application.

Section 4: documents you should not send unless specifically requested

This page provides guidance on the types of documents that are not required to consider your application.

• if you are applying as a family/group you do not need to provide multiple copies of the same documents
• bank statements or letters issued more than 1 year before the date of application
• driving licence
• photographs (other than passport photographs required in section 1)
• notarial certificates
• business cards
• hotel bookings
• flight bookings
• photocopies of bank cards
• credit card statements
• certificates relating to leisure activities
• evidence of car ownership
• travel insurance
• sponsor’s utility bills
• sponsor’s council tax bills
• educational certificates (unless specifically listed in section 3)

Emphasis added.

Source: https://www.gov.uk/government/publications/visitor-visa-guide-to-supporting-documents

Since you should not send a hotel booking, you certainly should not send evidence that you've already paid for your room. You are advised against paying, or making a nonrefundable reservation, because of the possibility that your visa application will be refused.