As others have said, there isn't enough information here for meaningful advice.
What I'm hearing is a sense of injustice: some employees seem to use this space without any problem, but you (OP) have not only been told not to use it, but told that doing so would be contrary to the employee Code of Conduct.
So one question is: what part of the code of conduct are you being threatened with? Failure to follow a reasonable management instruction is a common provision in such a code. Or perhaps the code even has a provision dealing with car parking. Either way, we need to know what it says and what exactly has been said (verbally? in writing?).
Is there anything in your contract of employment relating to car parking? Or anything in corporate policies about it? Employers have wide latitude for controlling employer behaviour contractually. There is nothing in principle, as far as I know, that would prevent them from instructing employees where they were allowed to park during working hours, and they may be able to do so outside working hours if doing so would damage or be a risk to the company somehow. But we don't know whether there are real issues about parking where you want to park, or whether management are making it up for some reason.
If there are lots of employees, I could imagine car parking in the area being controversial with local residents or visitors, and complaints from them might be a reason for a policy. And it seems to be that it wouldn't be an unreasonable management instruction to ask employees not to park in such a way as to cause complaints.
And these other car parkers? Do they have allocated spaces too, or not? I could imagine a company telling employees to use their private car park during working hours, if they have a space allocated, so as not to take spaces away from either members of the public or other employees who don't have an allocated space.
"Unfairness" isn't necessarily something you can take an employer to employment tribunal over. Not all unfairness is unlawful - you'd need to show that you've been treated unfairly because of a protected characteristic. For example, being able to park outside the car park but closer to the exit might be a reasonable adjustment due to disability.
But it could be grounds for internal grievance if a policy is being applied inconsistently (but we don't know if there is such a written policy, or that it is being applied inconsistently - the sparse info provided leaves this open).
