At the present time 'Prenumptual Agreements' are not legally binding under UK law. They 'may' be taken into consideration by a judge during divorce procedings, however all current legislation overides the agreement.
There are plans for Prenumptual Agreements to become legal in the UK, but they won't be a simple and easy way out of avoiding handing over everything. Also they will require your fiance to see an independant solicitor to give her advice on the agreement first.
Without the marriage, you will not be able to apply for a residency visa for your fiance, but you can in the meantime apply for a British Birth Certificate and Passport for your son, which you should do within 12 months of his birth. If you would like to know the procedure for that, I'd be happy to share it here on a seperate thread, as I've just been through that.
