There are a couple of things wrong with the way you structured your code (as
shown above)...

The first thing wrong with your code is that you need to pass the PostCode
into the function (otherwise the function doesn't know what to test).  I'm
not sure where you are getting your PostCode from within your program. If it
were from a TextBox (named, say, TextBox1 for this example, then your
If-Then statement should look like this...

If ValidatePostCode(TextBox1.Text) Then

Notice I did not set it equal to True or False. Why? Because the function
already returns either True or False, so asking if True = True or False =
False is redundant. That means, then, that the code directly under the
If-Then statement should be for the "True condition" (the "Valid" for you
example code) and the code after the Else statement should be for the "False
condition" (the "Invalid" in your example code).

The second thing wrong with your code is that you are trying to assign
String values to the function... you can't do that. In you have to use a
variable with a name different from the function, say,
ResultFromValidatePostCodeFunction. This would mean your example code above
should look like this instead...

If ValidatePostCode(TextBox1.Text) Then
 ResultFromValidatePostCodeFunction = "Valid"
Else
 ResultFromValidatePostCodeFunction = "Invalid"
End If

Of course, a variable named ResultFromValidatePostCodeFunction is kind of
extreme, but I used that to give you the general idea of how to approach
using the function.

Okay, now that that is straightened out, I have modified my ValidatePostCode
function to validate the PostCode Area as well as the "shape" of the Post
Code itself (which is all the original function did). Of course, this makes
the function less general than my original function meaning if any PostCode
Areas are added or removed UK Mail System in the future, the function will
need to be changed to account for addition and/or removal. Anyway, you can
stay with my original function or use the modified function below, your
choice.

Rick

Function ValidatePostCode(ByVal PostCode As String) As Boolean
 Dim Parts() As String
 PostCode = UCase$(PostCode)
 Parts = Split(PostCode)
 If PostCode = "GIR 0AA" Or PostCode = "SAN TA1" Or _
   (Parts(1) Like "#[A-Z][A-Z]" And _
   (Parts(0) Like "[A-Z]#" Or _
    Parts(0) Like "[A-Z]#[0-9A-Z]" Or _
    Parts(0) Like "[A-Z][A-Z]#" Or _
    Parts(0) Like "[A-Z][A-Z]#[0-9A-Z]")) Then
   ValidatePostCode = (Parts(0) Like "[BEGLMSW]#*" Or _
                       Parts(0) Like "A[BL]#*" Or _
                       Parts(0) Like "B[ABDHLNRST]#*" Or _
                       Parts(0) Like "C[ABFHMORTVW]#*" Or _
                       Parts(0) Like "D[ADEGHLNTY]#*" Or _
                       Parts(0) Like "E[CHNX]#*" Or _
                       Parts(0) Like "F[KY]#*" Or _
                       Parts(0) Like "G[LU]#*" Or _
                       Parts(0) Like "H[ADGPRSUX]#*" Or _
                       Parts(0) Like "I[GPV]#*" Or _
                       Parts(0) Like "K[ATWY]#*" Or _
                       Parts(0) Like "L[ADELNSU]#*" Or _
                       Parts(0) Like "M[EKL]#*" Or _
                       Parts(0) Like "N[EGNPRW]#*" Or _
                       Parts(0) Like "O[LX]#*" Or _
                       Parts(0) Like "P[AEHLOR]#*" Or _
                       Parts(0) Like "R[GHM]#*" Or _
                       Parts(0) Like "S[AEGKLMNOPRSTWY]#*" Or _
                       Parts(0) Like "T[ADFNQRSW]#*" Or _
                       Parts(0) Like "W[ACDFNRSV]#*" Or _
                       Parts(0) Like "UB#*" Or _
                       Parts(0) Like "YO#*" Or _
                       Parts(0) Like "ZE#*")
 End If
End Function