Ah, so a *useful* post for a change... ;-)

More intresting info in this thread - http://london.pm.org/pipermail/london.pm/Week-of-Mon-20030331/018067.html - including the suggestion that SAN TA1 might be a valid postcode. Is this true? I certainly hope so.

You can download a full postcode file from here:
http://www.brainstorm.co.uk/uk_post_code_search.htm

And there are some surprisingly interesting postcode related stats here:
http://neighbourhood.statistics.gov.uk/Default.asp?nsid=false&CE=True&SE=True

See also http://www.govtalk.gov.uk/gdsc/html/noframes/PostCode-2-1-Release.htm for even more details on vlid postcode formats.

mysql> select mf_outcode from PAF_Y03M10.fpmainfl where mf_outcode="S" or mf_outcode="SA" or mf_outcode="SAN" limit 5;
Empty set

If it exists, the grown-ups aren't meant to know about it either. ;)

There is one special postal code according to the Royal Mail's PDF, although it escapes me now. I think it was "GYROBANK", or "GYRO0000", or similar. If you are really interested, look at the London PM thread, I mentioned it there.

The special case that I know *exists* is "GIR 0AA" - this is indeed for the Gyrobank.

The post office does do special things with letters addressed to santa, usually sending them to a certain place (it chances every yeah) but I have no idea whether SAN TA1 actually exists. I don't think it does, though.

There are some notable exceptions to the postcodes that that list doesn't give: BFPO which is a prefix for British Forces (and goes somewhere in London) and BX1 something which is a special postcode owned by Lloyds TSB and goes somewhere in the south west.

If postcodes and regex are your thing, try

http://www.regexlib.com/REDetails.aspx?regexp_id=695