Re: Can they sack me?
If you're trying to find the phrase, "we reserve the right to terminate your contract if you park on the side street" then most probably you won't be able to find it. However, there could be something more general like the contract can be terminated for any reason etc.
Even if there isn't anything like that, they can make any number of (political) excuses like over-staffing etc to terminate your contract without getting into any sort of legal complications. Why would you want to work for a company anyway where there will be bitter feelings over the parking issue even if they won't be able to terminate your contract? The best option is to find a different parking solution that's convenient instead of getting into a confrontational situation.