It can vary from lender to lender. Most mainstream lenders have a ceiling of 75% LTV (Loan to value) requiring 25% deposit. These include all the high street lenders. Those that allow lower deposits are specialist lenders, so expect to pay a higher rate.
There are only two lenders in the market that allows purchasing a buy to let with a deposit of 15%, that is they would advance 85% of property value. There are many more for 20%. Of the mainstream lenders 20% deposit is restricted to how green or energy efficient the property is. That is only for properties with EPC ratings A-C.
The level of deposit also affects the rate, fees and the maximum loan amount. Larger the deposit lower the rate, as it is less risk to the lenders. Just because the loan is 85% does not mean the loan will be higher than a lower LTV. Because the calculation for maximum loan is not straightforward with Buy to Let lending. You can use a tool such as the https://landlordloanexpert.com/btl-loan-calc/ to obtain various