I've told them I want to keep my existing internet connection, that this is additional, but that legacy phone lines can be taken down. I want my Vodafone cable to keep working until I know the Snap connection's reliable.
Signing up online was easy, though support had to tell me how to find it - the main part of the website isn't very intuitive and doesn't have much information, which they tell me they're addressing. All in all I had to send support around 5 emails to get the information I needed to decide to sign up, which took around 11 days.
One disappointment is their password field is limited to 16 characters, and won't take the space character. I can't think of any obvious reason to impose those limits. Not a big deal.
This thread will record the installation process.