In England and Wales, you can leave your estate to whoever you like, there is no obligation to include any relatives / natural heirs.
In other parts of Europe, and Scotland, you cannot disinherit heirs completely, the advice is to leave a token amount and an explanatory letter. Some English lawyers think this useful, as it avoids 'challenges' that can be expensive even if futile.
Although DIY wills get a bad press, I think them quite sensible if your estate is simple. I have had 4 wills during my lifetime: 2 were DIY, done before I had children, then again when they were adults. When I had young children, and now grandchildren and their parents to consider, I have paid a lawyer to do more complex wills.