Do I have to pay the full fees before applying for a UK student visa or only a deposit?

You do not need to pay the full tuition fees before applying for a UK student visa. Universities ask for a deposit first to confirm your admission and issue the CAS (Confirmation of Acceptance for Studies). For the visa, you must show funds for any remaining unpaid first-year tuition fees mentioned on your CAS, plus the required living expenses. So if you have already paid a deposit, that amount is usually reflected in the CAS and reduces the funds you need to show.