Most people give the wrong advice on this subject.
You can get both income tax relief on your later mortgage interest and use the cash raised for anything you please since by buying the property for cash, your rental business owes you that money back and a loan can be raised to rectify this position.
It almost defies logic but HMRC actually confirm it and it is less understood by tax advisers than accountants because of the accounting nature of the capital account transactions and the use of a balance sheet.
Please see example 2 in this HMRC note... https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim45700
HMRC did actually change their guidance on this a few years back but the BIM45700 note still exists. Some tax advisers have misinterpreted the new guidance believing new mortgages must be wholly and exclusively for the purpose of the property business but what they don't realise is that the purpose of the loan to pay capital back to the owner is wholly and exclusively for the purpose of the property business because the proprietor of a business may withdraw the profits of the business and the capital they have introduced to the business.
The winning arguments are discussed here very well. https://www.accountingweb.co.uk/any-answers/remortgage-interest-allowability-0
I'd suggest you show these notes to your tax adviser/accountant or if you do not have one get one because HMRC might try to fight you on this.
Wright Tax Consultancy Ltd