You are not a resident of the UK unless you intend to stay there permanently, or return to the UK after being away.
What makes you a resident of a state (or country)?
Generally, you're a resident of a state (or country) if you intend to either stay there permanently, or return there after a temporary absence. It's where home is – where you come back to after being away on vacation, business trip, overseas employment, or school. Many factors are considered, not the least of which are where you are registered to vote, own homestead property and are licensed to drive.
Even if you were a resident of the UK, as a U.S. citizen you would still be subject to U.S. tax laws, including the requirement to report foreign bank accounts.
You would use Form 8938 to report your specified foreign financial assets, but only if the total value of all the specified foreign financial assets in which you have an interest is more than the appropriate reporting threshold.
If you are not married, the reporting threshold of your specified foreign financial assets is "more than $50,000 on the last day of the tax year or more than $75,000 at any time during the tax year."
For more information, see: https://www.irs.gov/pub/irs-pdf/i8938.pdf