When filing your taxes you want to use your address where you are most likely to receive your mail for the next few months. If you are only temporarily in the UK and have someone checking and accessing your mail at home, you would be best to use your USA mailing address. The IRS uses the mailing address on your tax return for any correspondences until you file a new return or update your address with them.
If, you do not have access to your US mail and will have your UK address for the rest of the year, then you would want to use your UK address.
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