Do I need a 1099 form for commissions over $600 paid to a contractor living in the UK?

Yes, if you want to enter the commissions as a deduction on your income tax return. You should have had the UK contractor fill out a W-9 form. 

he TurboTax Self Employed software allows you to prepare 1099 forms using Quick Employer Forms . You can access and create your 1099s in TurboTax, on the Welcome Back screen. 

• See What is Quick Employer Forms?

Per 2018 1099-MISC instructions Specific Instructions File Form 1099-MISC, Miscellaneous Income.

• Report on Form 1099-MISC only when payments are made in the course of your trade or business, for each person to whom you have paid at least $600 for services performed who is not your employee (including parts and materials).Enter the amount in box 7, Non-employee Compensation. Personal payments are not reportable.