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Maybe i am misunderstanding but that is the country code for the united kingdom and is one of the most popular in the world
This does include .co.uk .org.uk but the reason those exist is because you could not claim a pure .uk cTDL before 2014 because and this is an important piece of information for anyone looking to get a domain name. Countries can set the rules for their domain themselves; There have been instances (fmhy.ml) of governments revoking a registered domain, in this case because they discussed piracy.
You usually only need to specify the internal host ip to setup a port forward. It should forward that to whatever the public ip is at the time.
If the isp is providing the model/router and generally being oppressive i highly recommend researching if you can place your own router behind it.