Generally speaking, First-Class Package International Service from the US to the UK can take anywhere from 7 to 15 business days for delivery. This timeline is an estimate provided by the United States Postal Service (USPS) and is subject to variations based on individual circumstances. In some cases, packages may arrive earlier than the estimated time frame, while in others, they may take longer, especially during peak shipping periods or if there are customs delays.
To ensure timely delivery, it's important to use the correct address format, including the postal code, and adhere to any customs regulations or restrictions for the items you are sending. You may also want to consider using tracking services or registered mail to monitor the status of your package and ensure its secure delivery.