Earlier quoted context omitted.
I mostly went with them because of their privacy package. They do simplify the process a little bit by putting a nice UI on top of this more manual process [1]. I'd rather pay a few extra pounds than risk submitting wrong information thus creating more trouble further down the road. Perhaps, FUD was playing a role too. [1] - https://www.gov.uk/limited-company-formation/overview
I found it pretty easy when I did it three years ago, it's not hard. That said, I didn't need to bother, as part of on-boarding with my accountant they would have done it all for me.
My accountant also would have done it, but I wanted the company in existence quickly so I could get contracts signed.