UK Map Chart by Postcode

Hi All, trying and failing on a project here!

Trying to generate a heat map chart in excel broken by postcode area (AB, AL, BB, BS, BL etc.)

I've managed to get it working on a county level (Derbyshire, Kent etc) but cannot fathom how to break it down by postcode. Tried a few online templates and had no joy

See attached what I've managed so far, can someone give me a few pointers on how to go down another level and create this for postcodes?

Many thanks,
Andrew