Grounds for challenging the validity of a Will in UK law include lack of capacity, undue influence, fraud, and failure to comply with legal formalities. Lack of capacity refers to the deceased's inability to understand the nature and effect of their Will. Undue influence occurs when someone exerts pressure on the testator to make a certain provision in their Will. Fraud involves deception or misrepresentation in the making of the Will. Failure to comply with legal formalities means the Will does not meet the requirements set out in the Wills Act 1837.