If you want an interest free loan of circa £85,000, the answer is simple. It doesn't exist!
The last time I looked (a while ago) BBVA was offering secured money by way of a second property charge at around base plus 2.5% That is around 3%. You would be very hard pushed to find even a first mortgage rate at these levels nowadays. I recently saw an HSBC offer (to those with around 40% equity) of 2.99% (effectively the same rate) albeit with other fees.
BBVA are specialists in this type of loan, but you are right to shop around. It should also be remembered that as with all lenders, BBVA have tight qualification requirements for any potential loans. A lot of people will have difficulty satisfying those requirements.
There may be other things to also consider such as up front arrangement fees, early payment penalties, and graduated repayment schedules, The latter may not be available with most types of commercial products.