I am not familiar with family laws in UK but I am aware that in certain countries the prenuptial agreement is limited over a period of time, plus in certain countries the wife, despite of any agreement is entitled of a "legitimated portion" of your estate/assets.
Even with a prenupt agreement, a judge may overturn it. I give you an example, a man, married with a prenupt fell seriously ill and the wife spent years of her life to look after him. After his death the wife took the case to court and the judge ruled in her favor for some compensation.
There is not a clear cut in between.
For some, this may suggest some sort of distrust or lack of confidence on your spouse.
I would say a family lawyer can offer the best advise for your case.