ok....well the thing is as someone has already said, most registrars won't marry you unless you can prove that you are here legally and as you don't then there is the first thing. You have to get him legal.
What kind of visa do you have or are you naturalised here??
The best thing that you could do is to try and go to an immigration lawyer here and ask for the advice, to ask for a ILR (indefinate leave to remain) as a spousal visa or as someone you intend to marry. You can find alot of advice from the home office website about the things they grant people who are here legally BUT it does say that anyone who is here ILLEGALLY is not granted this, as they indeed have to.
The lawyer will probably find a way to try and ask the courts to grant "mercy" for you. The thing is, that if he leaves the country, one way or the other the authorities will find out that he has been here illegally and they don't look on it favourably. If he has his own money to leave the country then it is a bit better, but if he has to get the state to deport him, then it's a no go on him ever coming back here. You're in a bit of a vicious circle here, as if you want your re;ationship to continue then you need to be legal, but if they decline it and deport him, the it is over but if you dont go legal then there is always the threat of him being deported hanging over your head...
only real legal advice will be able to help..
can you tell that I have been here before??? 
and rainbow - it is very easy for him to pay taxes if he has a n.i. card and it hasn't been checked by his employeers for ages then he is paying taxes fair and square like anyone else.....