I have had similar issues! In our case, a mandatory field, Geo Region, needs to contain a value. This value is determined based on country, and is filled in within another smart campaign. However, if this smart campaign did not finish running in time before the person attempted to sync with SFDC, I would receive a similar error.
My recommended solution for you:
1. Update your sync smart campaign to start with a 1 minute wait step in the flow (before the sync step)
2. Add a trigger smart campaign to run when lead is created (trigger) -OR- when data value changes, attribute: country (trigger) -AND- country is UK (filter). For the flow step, update data value if postal code is empty to a placeholder.
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