I've fitted a thumbturn to my front door. In that case it was a ABS cylinder, though the Ultion was a close second. The ABS thumbturn is spring-loaded so you have to push it in before it'll turn. It's not completely infallible, but makes it much more difficult to stick a coat hanger through the letterbox. I worked on the basis that my priority was my family getting out quickly in the case of a fire, without having to hunt around for keys.
I phoned my insurance company (Direct Line) and told them, and requested written confirmation that they were happy with this arrangement. They sent me a letter in the post confirming that they are "aware that the front door is fitted with a safety feature that allows egress without the use of a key" and confirmed that this doesn't affect the premium or level of cover at all.
Your insurance company may say the same, but given that you no longer have "key operated locks on all external doors" it's best to tell then.
I fitted a GRP Door Stop door on my last house. I wish I could have brought it with me. I loved that door :-)