Prenuptial agreements (PNAs) are not formally binding in England and Wales but are regarded by courts as persuasive and often decisive in many applications for financial relief following the breakdown of a marriage.
Whether or not the PNA will be upheld in court, will depend on the court’s view having regard to a number of factors. If done properly, you can expect the agreement to be fully taken into account and it’s terms upheld.
Since the landmark Supreme Court decision in Radmacher v Granatino 2010  UKSC 42 the use of PNAs in the UK has grown rapidly. In Radmacher, the court said:-
“The court should give effect to a nuptial agreement [PNA] that is freely entered into by each party with a full appreciation of its implications unless in the circumstances prevailing it would not be fair to hold the parties to their agreement”.
In order to persuade the court to uphold the agreement, the following should be dealt with:-
a. The Agreement must enable the parties and children to meet their needs and it must be fair. This is perhaps the main and most important clause. If the agreement does not enable the parties and children to meet their needs following a divorce, then the court are highly unlikely to uphold the agreement, even if all of the other formalities have been met.
b. Parties must sign of their own free will. The parties must enter into the agreement of their own free will and there must not be any undue influence or duress to sign the document.
c. Timing. Along with there being no duress, it is also essential that both parties have sufficient time to consider the terms of the document. A PNA should be signed at least 28 days prior to the wedding. If you are contemplating a PNA, It is therefore good that your Dad is seeing someone today.
d. Financial disclosure. Both parties will need to provide full disclosure of all of their financial circumstances before the agreement is signed.
e. Legal Advice. Whilst this is not mandatory, it is highly advisable that both parties should take legal advice at the time agreement is drafted and sign a declaration confirming that they fully understand the document and its implications.
f. Contractual validity. The agreement must be capable of being enforced as a contract.
g. Execution. The agreement must be executed as a Deed.
In the circumstances, your Dad should progress with it and it should be fine.
Best of luck!