After getting the UK offer letter, do we have to pay tuition fees to the university to get the final visa?

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.