You did the work in the UK, so it's taxed as self-employment in the UK. Sign up for self-employment with the HMRC. It's on their website.
You will of course ALSO have to report the income in the US, and can claim either the 2555, the 1116, or a combination thereof to adjust for the UK taxable element. You will NOT owe US SE tax (self employment or social security tax) as you are paying it in the UK.