RE: Is it possible that in environment have two companies. One in USA and one in UK
Hi, as far as I know, there is no way so far.
There is a big limitation here, the local Localization Pack cannot be installed in the environment of other regions. For example, J-Pack, which I was in charge of in Japan, can only be installed in the environment where the Region is JP (The language pack is no longer limited). I discussed this problem with Søren, the person in charge of BC Localization at Microsoft, and Microsoft believes that if the Localization Pack can be installed in other regions, then this app not a local package, but a universal package.

And as you know, Apps in an environment are universal, we cannot install Apps for a certain company. I haven't studied the UK version, but I know that the tax treatment of the USA is very different from the W1 version. So this requires a lot of testing and customization if the local packages are all in one region.
More details: Local Functionality in Dynamics 365 Business Central
https://learn.microsoft.com/en-us/dynamics365/business-central/about-localization
Just share some info, hope this helps.
Thanks.
ZHU