To do this, the 2 controllers would have to be in separate groups. Also, the Global Aruba Config setting on AMP Setup would have to be set to 'no'. In this way, your UK controller would sync with the UK group Aruba Config, while the US controller would sync with the US group Aruba Config. Let me know if you need a walkthrough for setting this up.
if starting from scratch:
1. go to AMP Setup, disable Global Aruba Config
2. create 2 groups: UK, US
3. add UK, US controllers into their designated groups (in monitor only mode)
4. for each group, navigate to controller's config audit page and import settings / profiles
5. check and confirm that the RAP whitelists you desire are separate by verifying on each group's Aruba Config tab
from a pre-approved point (if devices are already in AMP):
1. make sure controllers are in monitor only mode
2. blank out RAP whitelist (show all entries in the table, then select all, delete, save)
3. create new group for one of the controllers
4. leave whichever controller you like in the current group, while moving the other into the new group
5. for each group, navigate to controller's config audit page and import settings / profiles
6. check and confirm that the RAP whitelists you desire are separate by verifying on each group's Aruba Config tab