Are you a UK resident now and self employed in the UK? Or has your freelancing work been done by your US employer while living in the UK?
If you're a US citizen who is a UK resident and self employed, your taxes get pretty complicated. It depends on if you're a sole trader or limited company and I highly recommend hiring an accountant to make sure you get it right in both countries. I just started as self employed in the UK and I tried for months to figure things out on my own and it's nigh on impossible. The really simplified version is that the US and UK both define self employment differently and that makes some laws written to prevent double taxation irrelevant. I've found an accounting company that seems to know what they're doing, but I've hired them for next year's taxes, so I can't really speak for them yet. I'd be happy to PM you their info if you're interested.
If you worked as a freelancer in the States before moving to the UK and have had regular, taxable jobs in the UK since moving, then I think you just do whatever you normally would need to for self assessment. But I'm not certain on that at all, definitely check with someone with more experience than me!
