Q: Am i right in thinking that once i have downloaded my photos and videos to my computer by using the icloud portal they are then safe and if i  then delete them off my phone  I haven't lost them forever ?

I assume you are referring to:

iCloud Photo Library - Apple Support

where it says:

Back up your photos and videos

When you turn on iCloud Photo Library, your photos and videos automatically upload to iCloud. They're not duplicated in your iCloud backup. But we always recommend that you keep back up copies of your library. You can download your photos and videos from iCloud.com to your computer and store them as a separate library, transfer them to another computer with Image Capture or Photos, or store them on a separate drive.

I would recommend using one of those 3 options.