Week number - first full week rather than 1st January?

First let me remark that I have never heard about such a system of week numbering.

With ISO, the weeks start on Monday and the first week of the year is the first Thursday of the year.

Or, in other words: Jan 4 is always in week 1.

If Jan 1 is on Thursday, then the week from Monday Dec 29 thru Sunday Jan 4 is ISO week 1.

If Jan 1 is on Friday, then the week from Monday Jan 4 thru Sunday Jan 10 is ISO week 1.

The following code gives the week number according to your definition:

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date", type date}}),
    #"Inserted Start of Week" = Table.AddColumn(#"Changed Type", "Start of Week", each Date.StartOfWeek([Date],Day.Sunday), type date),
    AddedJan1 = Table.AddColumn(#"Inserted Start of Week", "Jan1", each #date(Date.Year([Start of Week]),1,1), type date),
    AddedWeek = Table.AddColumn(AddedJan1, "Week", each 1+Number.IntegerDivide(Number.From([Start of Week] - [Jan1]),7),Int64.Type),
    #"Removed Columns" = Table.RemoveColumns(AddedWeek,{"Start of Week", "Jan1"})
in
    #"Removed Columns"