You cannot force your partner to enter into a pre-nuptial agreement. If undue pressure is applied it will most likely make the agreement unenforceable. However, it is also your choice as to whether or not you get married. By marrying in England and Wales you are consenting to the courts determining how your assets are divided (if no agreement can be reached) upon divorce.
Please note that you cannot contract out of any child maintenance regardless of your marital status or whether you have a pre-nuptial agreement.