My wife and I are US citizens and got married prior to moving to the UK. Before getting married we signed a prenuptial agreement. If we become UK citizens will the prenuptial agreement still be valid? What happens if we, or one of us, renounces our US citizenship?
There are likely to be two basic problems that will come into play in these cases, namely choice of venue and conflict of laws. These aren't necessarily problems, but they are things to talk about with a lawyer if you are concerned. These problems occur with international contracts generally and so lawyers are likely to be in a good position to help (i.e. ask a lawyer).
Basically, the question is "who interprets the contract and according to which rules?" In theory, different jurisdictions may interpret contracts entirely differently. In practice the differences are more narrow, due to the overarching needs in international business for consistency in contracts.
These could still impact you however. Jurisdictions tend to read contracts in line with questions of public policy, and these considerations do differ from one jurisdiction to the next. A court in the UK may read the contract in line with different approaches to family law than the US would. I would generally expect the differences between most of the US and the UK to be less than between the UK and France, since France is civil law based on Roman tradition, while the UK is common law based on a mixture of Anglo-Saxon, Viking, and old French tradition (and thus are structurally different). But that same difference holds between Arkansas and Louisiana.
The major complication of course is where the case gets decided. If you split up, then you have the possibility that one of you may choose to stay or move and litigate the divorce in whichever jurisdiction you think is most advangageous. To some extent that undermines the idea of shared expectations in that case that a prenup is supposed to help with.
So I don't think it would be invalid but there are reasons to talk with a lawyer here.
The generic issue is as I stated in the comment the application of the prenuptial agreement signed in another country and usually governed by the laws of another country to the divorce proceeding and law in the country where you have filed for divorce.
In the UK prenuptial agreements carry less weight then they do in the US. So if the judge chooses to apply the prenuptial agreement (read contract) the judge will apply the contract. Your citizenship in this case is not relevant since you (or your wife) are still the person that signed the contract.