What part of marriage makes it legal? Is it that the registrar has to be, ahem, registered? Or is it the actual signing of the marriage licence? I know someone who wants to be married in everything but law for financial reasons, and would this in itself be illegal?
The registrar acts as a legal representative, however, the signing of the marriage licence forms a legal contract between the two parties 'at law'. Your friends case would not be illegal because it would not be legal! And pre-nuptials don't apply in English law. They might consider getting married in a foreign country like Las Vegas (kidding). One can not be married in everything but law... You can't have your cake and eat it too.
Theres no such thing as being married in everything but law - its a legal contract (even if they are married abroad). However, I understand where you are coming from. The British Humanist Association will perform a 'marriage' with all the trappings of a marriage ceremony but it does not have any legal authority. It may satisfy their friends and family. Check out their website)
ps I'm intrigued to know what financial constraints your friends have to getting legally married.