There is nothing technically to forbid you doing that, each node in the DNS tree is the apex of the whole zone below it.
But there are not only technical reasons.
First, the problems with "levels" is that it depends from the registry. Historically, you were only able to register
.co.uk domain names because this is what the registry (Nominet) decided to allow, which is not uncommon among ccTLDs even if it becomes the exception. Now things have changed, and you can register directly under
Second, each registry decides what domain names are allowed or not in the TLD it provides. Each registry has it list of reserved domains (not allowed at all) and/or premium domains (available under certain conditions such as eligibility rules or specific prices).
You should have asked the registry directly why the specific domain of your wishes is not available.
For gTLDs you can find from example here: https://www.icann.org/resources/pages/reserved-2013-07-08-en domain names reserved in all new gTLDs, and in the past, registries had prohibition to allow registration of other gTLDs as labels in their own gTLD, but the set of them was small.
.co.uk if you read the Rules of Registration at https://nominet-prod.s3.amazonaws.com/wp-content/uploads/2015/10/Rules_June_2014.pdf section 5.6 clearly states:
For a combination of policy and continuing technical reasons the Characters "com" and "uk" shall not be permitted as an SLD or a Third Level Domain within co.uk, me.uk, org.uk or net.uk.
So, until the registry decides to amend their rules, your wish will never be granted.
Now the (rhetorical) question would be: why do you care so much about this particular domain name, and why not settle for any other one?