Saltstack Jinja template pillar merge
The "saltey" way of doing this is typically in a map.jinja file. This one for example...

In that, you would:

{%- load_yaml as serverlist %}
#Note that a default list could be provided here
{%- endload %}

{%- set serverlist2 = salt['grains.filter_by'](serverlist, merge=salt['pillar.get']('pillarname:server_list_de')) %}
{%- set servers = salt['grains.filter_by'](serverlist2, merge=salt['pillar.get']('pillarname:server_list_uk')) %}


Then in your sls file you would then put at the top:

{%- from "formulaname/map.jinja" import servers with context %}


Servers should then be available to you. I've never tried merging twice though, so give it a try. At a minimum this may give you some direction as the syntax for doing a pillar merge like this may get you what you are looking for.