I'm now offering a UK postcode data file containing the longitute and latitude for full UK postcodes (not just outfields). Here's a sample:
As you can see, both infield and outfield are given, and there are just under 2.3 million records in total, making it extremely comprehensive.
The data is around 3 months old now, and was compiled by myself (ie it's not just a copy of the Royal Mail's PAF file).
If anyone would like a copy, the price is 80 pounds. My email is firstname.lastname@example.org