I'm an American citizen but reside in the UK. I'd like to do some freelance work for a company in America, but don't know the tax legality/implications.

[viktoriyab11]

1. Will you work as an independent contractor or an employee? If you’re an independent contractor, you get a 1099-MISC form. If you’re an employee, you receive a W-2. As a W-2 employee, payroll taxes are automatically deducted from your paycheck and then paid to the government through your employer. If you’re a contractor, you are responsible for calculating your own payroll taxes and then submitting the sum to the government on a quarterly basis. 
2. You have to follow up with IRS filing requirement for U.S. Citizens and Resident Aliens Abroad. This link will guide you. https://ttlc.intuit.com/replies/6453872