Does an illegal immigrant (‘overstayer’) have legal rights?

Can they transact a legal transaction or will the transaction be void given their criminal status?

Also, if I have reason to believe that they are an ‘overstayer’, how can I find out?

Thanks

My name is Amaaley.I live in uk and I am indefinite holder. I was married with someone who lives in syria but now I am divorced from him and I don't have any latter or prove about my divoerce. So, I would like to know if I can get married again without that letter?

And what will I be asked if want to get married and register myself and my future husband as he has a student visa?