Just a few penniesworth from me on the general question raised at the start...
In England & Wales there is "freedom of testamentary disposition". This means that you can leave your estate to whosoever you wish, including of course non-family members, charities and pets -- provided you create a trust for them, you could leave a bequest to, say, your parrot. A Will can, of course, be contested but good cause would have to be shown to vary its terms. These days there is also an act called the Inheritance (Provision For Family & Dependents) Act which makes certain classes of claim more likely to succeed -- e.g. claims from cohabitants and financial dependents.
In Scotland there are some restrictions on testamentary freedom. Spouse and issue have legal rights and cannot be disinherited.
Either way, many family members are excluded from Wills, whether because they are already amply provided for, or are independently wealthy, or have fallen out / emigrated / lost touch etc. Like every source of information, Wills are useful but not definitive.
Personally, my sense is that Wills are an under-used resource. I also think it is a misconception that poorer classes of people never left Wills -- certainly from the late Victorian period to date, in my inglorious family many persons with little more than a bean to their name did actually write a Will (or perhaps had it written for them).
