On youtube i have seen lots of TV Shows, like Air Crash Investigation, i downloaded one of these and re-uploaded it and under youtube's copyright they put ads on it, i am guessing this would go to the shows owners - National Geographic Channel? Also - if i was to embed these video on a site, can I, in that case the pre-roll ads would not run? I it legal?
Not sure I understand your question, someone else may. I just embed YouTube videos on my site as is. I reckon people understand that there are ads anyway. I however do keep the video intact.
I have uploaded a TV show and under copyright youtube place pre-roll ads onto it which is fine by me - BUT i would like to embed these video into a blog and these for the ads will no long play - is this ok?
Okay, you must keep them intact by law. Don't let that stop you using them if it's good info for your visitors. Look at it this way - people come to your site and see a relevant video that interests them. What do they do? Stay on your site to watch it! That is great for lowering your site's bounce rate and great for your rankings in the long term. Google love it when you add videos to your content. Read this post over at WarrioForum for more detail about your question. The only other suggestion is to download it, then make your own edited video introducing the rest of the video content. Refer to the source.