I took a quick look online and found this:
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Are they legal in the UK?
In the United Kingdom, pre-nuptial agreements currently have no legal standing. The divorce courts have the last word in the division of all matrimonial property and would ignore any pre-nuptial agreement if they thought that it was in any way unreasonable to either of the parties involved, particularly regarding to the maintenance and housing of children. For example, regardless of whose name a couple’s property is in, a court may order it to be transferred to the other. This applies to all property that is owned seperately or jointly
However, the rules for unmarried couples are quite different and legally binding contracts are enforceable so long as the couple remain unmarried. It is divorce that gives the courts jurisdiction to intervene and make orders. The widespread belief that long standing unmarried couples have rights as common law husband and wife is a myth."
Pre-Nuptial Agreements in the UK