English law provides that a testator can be capricious. We have no system of forced heirship, unlike other jurisdictions.
There are numerous ways of challenging a will: lack of testamentary capacity (the test is not the one under the MCA, but under a case called Banks v Goodfellow), lack of compliance with formalities, undue influence, lack of knowledge and approval.
There are also ways of mounting a claim against the estate such as under the Inheritance (Provision for Family and Dependants) Act 1975, proprietary estoppel.
These are not all the challenges that can be made but are the most popular. With a bit more information, I might be able to be more specific.