It sounds like you’re talking about freedeedpoll.org.uk? If so: no – to be witnesses to a deed, a witness must actually be present at the time that it is signed. Having witness signatures pre-filled would entirely undermine the point.
I’ve no interest in doing interviews or charging a fee: there are plenty of others – like the solicitors you mention – who cater to people who want to pay to change their name. I created freedeedpoll.org.uk to help people to understand that they don’t have to pay any money in order to change their name.
Strictly speaking, a deed does not require witnesses at all, and you’re welcome to make a “witness-less” deed poll and see if anybody will accept it. You’ll probably find that you have trouble! But if it works out for you: great! Witnesses provide a level of validity and protection, which is why they’re preferred. Similarly, the choice of witnesses can be important: again, there are few stipulations about who a witness can be, but choosing somebody who knows you well but not closely is ideal… but a friend, colleague, neighbour, etc. would probably be no problem.