Hi @liya101,
Looking at a couple past posts, it looks like there are ways to incorporate UK post codes, they may just not be as granular as preferred.
Have you set up a geospatial hierarchy for your field? Additional steps about setting this up can be found here:
I would also take a look at the geospatial troubleshooting page as well as that may be able to shed additional light on your issue.
Let us know if you have any additional questions