I use Fiverr.com to have a freelancer work for me. He lives in UK, do I need to send a 1099 to him when the work was in the UK but hired in US? How does this work?

As a user of Fiverr services, you do not need to send a 1099 form to your seller (contractor), even if he is a US person. In fact, the contractor relationship is between Fiverr and the seller, and Fiverr will send 1099 forms to US sellers.

Yo do not need to send a 1099 form to Fiverr either as they are a foreign company.