Unfortunately, things are not quite as simple as we'd sometimes like to think.
As I said, the short answer is that ’ukkāl is some sort of passive participle. The rest is not clear.
The most likely option is that it is an alternative passive qal participle. Perhaps a relic of an earlier form from before ’ākhūl came to be used, or perhaps some kind of parallel form. It also seems to be a relic of the same participle that became the base of the passive qal perfect.
Other alternatives that have been suggested are that it's an alternative form of the passive pi‘ēl participle.