A third level domain registration is not the same as a private subdomain, even though technically they are nearly the same.
Examples of valid third level TLDs: .com.mx, .co.uk, .org.uk, .co.nz, .com.au
Examples of private subdomains marketed as fully-fledged extensions (Centralnic in particular): .uk.com, gb.com, .de.com
The relationship is completely different.
.co.uk is not considered a subdomain of .uk, but an extension of its own and you are dealing with Nominet.
On the other hand if you buy a .uk.com then the ultimate owner is the owner of uk.com. Of course there are legal implications if things go wrong (and they have).
The biggest problem is probably confusion, if you are in Brazil and you use a br.com, people will think it's a typo of .com.br so you risk losing a lot of traffic.