OK, not quite a eureka moment, but the following should work:
It formats numbers in the format #.## or # in the third table column (set at
iCol = 3)
to use either GBP or EUR currency separators. Calculated fields are updated
and converted to text before formatting.

I have crossposted to the vba general forum in case someone there has a
simpler solution.

Sub FormatColumnAsCurrency()
Dim cTable As Table
Dim oRng As Range
Dim sNum, sCent, sEuro, sTEuro, sMEuro As String
Dim sSep, sDec, sCurr, sSign As String
Dim iCol As Integer
Dim i As Long
Dim count As Variant

'Set column number containing the amounts
iCol = 3

sCurr = InputBox("Enter Y to format table as Euro" & vbCr & _
                "Enter anything else to format as Sterling", _
                "Format currency", "Y")

Set cTable = ActiveDocument.Tables(1)
For i = 1 To cTable.Rows.count
   Set oRng = cTable.Cell(i, iCol).Range
   On Error Resume Next
   With oRng.Fields
           .Update
           .Unlink
   End With
Next i

For i = 1 To cTable.Rows.count
   Set oRng = cTable.Cell(i, iCol).Range
   sNum = Replace(oRng, Chr(13) & Chr(7), "")
   sNum = Replace(sNum, ",", "")
   If InStr(1, sNum, ".") = False Then
       sNum = sNum & ".00"
   End If

   If UCase(sCurr) = "Y" Then
       sSep = "."
       sDec = ","
       sSign = "EUR "
   Else
       sSep = ","
       sDec = "."
       sSign = "GBP "
   End If

   sCent = sDec & Right(sNum, 2)
   sEuro = Right(Int(sNum), 3)
   If sNum > 999999 Then
       sMEuro = Right(Int(sNum / 1000000), 3) & sSep
   Else
       sMEuro = ""
   End If
   If sNum > 999 Then
       sTEuro = Right(Int(sNum / 1000), 3) & sSep
   Else
       sTEuro = ""
   End If

   sResult = Replace(oRng, sNum, sSign & sMEuro & sTEuro & sEuro & sCent)
   sResult = sSign & sMEuro & sTEuro & sEuro & sCent
   oRng = Replace(sResult, Chr(13), "")
Next i
End Sub