I have used Lawpack: www.lawpack.co.uk/
They are very clear and easy to use, and step-by-step indicate when you need to go to a solicitor.
My own opinion is that if your will is very straightforward, then Lawpack are fine.
I have had 4 wills in my life: the first when just married, was a WH Smith form. The second and fourth, in which I needed to consider young children and grandchildren, were done by a solicitor. The 3rd, when my children were adult (but no GCs) was Lawpack.
I have helped to execute 2 wills. One was an old WH Smith, kept up-to-date, and was clear and easy.
The other, done by a solicitor but not kept up-to-date, was a nightmare, mainly because of tracking down people who were dead or had moved several times, including the solicitor's firm which had undergone a lot of changes!
We now review our wills every New Year!