It sounds like this person is a loan shark – or in other words an unlicensed money lender.
Before things get worse you need to contact the Illegal Money Lending team on 0300 555 2222 or email reportaloanshark@stoploansharks.gov.uk
They also have an online webchat here: stoploansharks.co.uk If you live in Wales, Scotland or NI, the contact details can be found here.